You said "Granted I was one of the self-motivated students while a lot of my classmates wasted time" which clearly points out that you probably didn't fall into the high percentage of students that can't resist doing things in class that they should be, while acknowledging that it wouldn't work for "a lot" of your classmates. There are a lot of rules/laws in society that effect 100% of the people, but are there to make sure that a smaller percentage of the population isn't hurt or taken advantage of in some way. In a perfect world where we have nothing but self motivated students who aren't easily distracted, there would be not be a need for filters. Even if it was just a small portion of the population, there could be alternative means to control, but with the vast majority of students it's just too much of a temptation not to screw around instead of doing what they are there for, which is to get an education.

Of course there are exceptions to every rule and we had them as well. There were times we allowed games in the classroom and we had the ability to turn them on for just single computers, a room full of computer, a whole school or anything in between. I would bet that if you were to ask your teachers if s/he would mind if their students as a whole were capable of free access to the Internet while they were trying to teach a class, they would almost universally say no, and the ones who said yes, would probably be low on the list of teachers if the principal were to rank them.

Imagine this, a very smart self motivated student finishes up whatever they are working on and the teacher allows them to play video games till the end of class. Meanwhile you happen to be the unlucky student next to them that had to deal with a video game going on out of the corner of your eye while you finish up your work. I would bet that you would find that situation more than just a little distracting... how would you like to get a C instead of an A because of being forced into situation...

I was the tech director of a school district for 13 years. I've run schools with very restrictive Internet filters and everything in between to schools with no restrictions at all. What I've found over the years is that the more you restrict the Internet the more the school's grade average goes up, and the nicer the students are to deal with. Our schools consisted of about 75% to 100% of the classes,depending on the school, being delivered though distance learning courses. If you give the kids open access to the Internet 90% of the kids will just chat, play games and watch non educational videos all day every day. They get away with this by leaving a window with their school work up and when the teachers comes to check on them they bring it to front, or by making the offending browser window very very small, so that you can't tell without looking very closely that they aren't doing your work. Left unchecked, at the end of the year, 90% of the students would need to be held back a grade. A couple of side effects of kids that aren't on task is they tend to have very bad classroom behavior that disturbs the students that are trying to stay on task, and most of the time wasters the kids like to use are also HUGE bandwidth hogs, so you end up having to buy 10X the Internet connection that you actually need for the school to function, which only deprives the school of much needed funds that could better be spend on something else.

The extreme other side of the coin, and the way the school is currently running is to completely block the Internet except for a select few websites that the school needs for their distance learning courses. There are some "research" or "library" computers that the kids need special permission to use when they need to look things up for papers and such. By blocking everything, the grade average of the entire schools district has shot up to record highs, and the classrooms are a lot more quiet and easier to control.
When it comes down to it, schools are a closed environment that is specially designed for education. When you introduce distractions into that environment that level of education that the kids are getting goes down significantly. It's not a matter of free speech or the school snooping in on private things, it's a matter of making sure that your kids get a certain level of education.
As for using school computers for personal activities and the school snooping in on them... you weren't supposed to use the computers for personal activities at all. Everyone, teachers and students alike, sign off on the school's computer use policy at the beginning of every year, and I don't know of a school that doesn't require one in some form. We didn't give the teachers computers so that they could maintain contact with their family while they were supposed to be working, and we didn't give the students computers so that they could keep in touch with all their friends on facebook. To argue that it is violating their rights not to be given unfettered Internet access would be like arguing that the school should provide every student with a cell phone so that they could keep in touch with their family and perhaps call people for help on research for papers... even if you could figure out a good reason to give students a cell phone, it would ultimately be a complete flop and a total distraction for an education environment.

In a traditional school, the students time on a school provided computer would be a lot less and therefore a lot less of noticeable
on their overall grades, but the problems are still there.

All that being said, I am completely against any kind of censorship when it comes to my personal Internet, or anyone else's personal Internet, but when you get into a school/business environment, it's no longer YOUR Internet and the owners of the Internet connection can do with it what they like... you have to remember, they don't HAVE to give Internet access at all, and whining that they are blocking access to things that are not in keeping with the task at hand... well maybe you should think about what you are saying before you start whining. After all, you are probably 1 step away from being expelled/fired, and the block is their way protecting you from yourself.