I actually work under a union (SEIU 503):
My union doesn't impose their views on me - I'm not sure how that would actually work. They do send out a newsletter - I can either read it or not. Contrary to what you may think they don't shout their beliefs over loudspeaker.
You can't do anything (really) to prevent someone from crossing the line, but you can make it more difficult. While your co-workers are out working hard to keep management from decreasing your pay you sit on your ass reaping the benefits.
On your last point - I hate people who reap all the benefits that we worked hard for in our union, but don't participate at all. If you don't like working in a union shop - leave the company - that's your right to work :).
Unions worked hard for the weekend - remember that any time you have a day off.