That is something you could say about almost any legislation, and it's correct to some extent. That said, politicians do seem to try and predict what they'll be held accountable for, and pre-empt that. So it's not so much the voting afterwards that potentially influences the behaviour, but the threat of doing so. I also assume that this is why contacting your representatives is important - that is, essentially, making the threat.

I'll give you that it's not a terribly strong signal.