It's done for a variety of reasons -- ignorance, special interests, or pandering -- but the same enabler -- not caring so much about the constitutionality of the laws passed.
Congress should have a legal duty to uphold the constitution.
I agree with the grandparent. It's the same thing here in Germany: The parlament has passed many, many unconstitutional laws throughout the last decade. Courts invalidated them later.
There are two main issues:
1. Those laws set expectations to what is acceptable. Constitutional but extreme laws later pass as "moderate" in the opinion of the public.
2. Respect for the constitution gradually erodes.
2. Lots of energy is wasted, because EFF type of organizations need to go to court, collect evidence, testify, etc.
I'll also add, it punishes citizens with jail time or fines for not following unconstitutional laws until they are invalidated...which takes a long time, at least in the United States.
So the desired chilling effect occurs regardless. In fact, the goverment can (and does, sigh) play the game like this:
1. Pass an unconstitutional law and enforce it.
2. Wait years for the inevitable legal challenge to make its way to the Supreme Court, at which time it is invalidated.
3. Change the law slightly (minor variations in wording are sufficient), pass it, and enforce the law.
4. Go back to step 2.
The government can do this as long as it wants, without any problems (i.e. there's no fine for passing unconstitutional laws, and those who vote for them don't lose office, etc.).
It would be interested to see what would happen if it was upheld that a sworn official who passes (or votes for) a law that is later deemed unconstitutional has to step down. I don't believe such a principle would ever be approved by official bodies, but it would certainly have an interesting effect, one way or another.
Congress should have a legal duty to uphold the constitution.