There is no law preventing a politician from lying to constituents. But what's depressing is that people who should know better believe them.
Walter Williams made an excellent point in Liberal Reasoning: Idiotic or Dishonest?. He points out several instances in which progressives -- he calls them "liberals" -- demonstrated that they understand that if you make some activity more expensive people will do less of it. Cigarettes taxes are a good example.
But when it comes to the minimum wage they suspend their belief. If they raise the cost of hiring someone, there will be less hiring. Here's Williams:
.You want less of something? Just raise its cost. ...
What the liberals believe — and want us to believe — is that though an increase in the cost of anything will cause people to use less of it, labor is exempt from the law of demand. That's like accepting the idea that the law of gravity influences the falling behavior of everything except nice people. One would have to be a lunatic to believe either proposition.
He asks in his title whether they are idiotic or dishonest. That's a fair question, but there's a third answer. The desire to feel good about themselves trumps logic.