I'm with the AC on this. The real question is "what are they paying you for?" If they're paying you for your time, then you owe them your time. If they're paying you for your talents, then you owe them your talents. If they're paying you for making sure work gets done, then that's what you owe them.
I couldn't care less what my employees do with their time so long as they accomplish the goals we set. I'm paying them to accomplish something in a certain period of time. I expect them to meet a certain level of professionalism, but beyond that, all I'm interested in is the work.
It works the same at my house. My kids have chores to do and a time period to get it done in. I don't care when they do them, so long as the chores get done in a reasonable period of time. When I pay someone to mow my lawn (rare) then I don't care if they're on their phone taking on other jobs or doing tech support for Comcast, so long as my lawn is mowed when I need it mowed.
Not every job is measurable like the ones I'm describing so I can't make a blanket statement that every employer should work like I do. Funny thing, there are lots of different kinds of people, lots of different kinds of jobs, lots of different types of agreements between employers and employees.