The problem with following their constituents' conscience is that their constituents do not have a single, unified conscience. Unless they taken an opinion poll before every vote, they are going to have to follow their own conscience. I don't really have a problem with that. I have yet to come across a politician (or anyone else for that matter) that I agree with 100% of the time, so I don't expect them to vote the way I would like them to every time.