Why do people think that being called "Liberal" or "Conservative" means a politician can only vote that way?

What right do we as citizens have to put people in little boxes labeled "Liberal" or "Conservative" , "Democrat" or "Republican" and expect them to stay in them? It is our right to vote as we see fit. We ought not vote for someone thinking we'll have control over them or any right to judge them, you're supposed to vote for people for things they agree with you on, and accept the parts they don't. It's called a compromise. No one is perfect and you can't reasonably expect them to agree with you all the time, especially since, being politicians, they probably have access to a good deal more accurate information, and more of it, than you do. Honestly, have a partisan system doesn't do much for anyone except categorically divide them, which can be useful from the perspective of differentiating them I suppose, but really, they're still their own person with their own beliefs to deal with. Democrat, Republican, Liberal, Conservative; they're all just empty titles in the long run.