Maybe I’m thinking too simplistically when I read the question posed by the OP, but the answer is really “because he can, untill somebody stops him.” Here is a more detailed version of Enginerd’s answer:
The OP seems to be predicated on the assumption that somehow the Constitution automatically prevents any branch of government from violating it. Unfortunately, it just doesn’t work that way.
Any branch of government or government entity, from the President of the United States to the Cabinet to the State governments to your local mayor or City Council or Sheriff or Judge can enact any law they want (within their power to do so, of course) whether it is constitutional or not. Even the most egregiously unconstitutional law will remain in effect until somebody sues them to stop it on the grounds that it is unconstitutional. When it gets to the court, the court can then rule whether or not it is actually unconstitutional, and if they rule that it is, only then does the law cease to be effective.
To complicate the matter even more, not anyone can just go to court to have just any law declared unconstitutional. As pravnik pointed out, you have to have what is called standing. That means your right to take someone to court. For the most part, you have to be “harmed” in some way by the law in order to take it to court.
For example, let’s say your local town council makes it illegal to put up a sign that says “The Mayor is a Buttwipe,” punishable by two months in jail. You might think that this is blatantly unconstitutional, because we have a first ammendment right to free speech. However, unless you have put up such a sign and been arrested, and then convicted, you would not have the standing to challenge the law on constitutional grounds. Even if the first local court decides that the law is unconstitional and doesn’t convict you (which is possible but unlikely, usually you have to appeal to a higher court for constitutional rulings), you still have to put up with being arrested, and hauled to jail and processed, and waiting in jail or out on bail until the trial, etc. Should no one want to put themselves through all that, the law may never get declared unconstitutional, as much as it may be.
Now all this is over simplified, I’m sure. I am not a lawyer and I’m sure many will jump in to correct me for what are sure to be many (hopefully) minor errors, but I think my point is correct.
The whole idea of the constitution is to protect the people against abuses of THE GOVERNMENT. When the government tries to go too far or if any agent of the government abuses the public, the constitution provides a way to stop them. But it is a process that can be long and drawn out and costly. And someone has to be willing to take the fight to the government to stop them, which may have its own perils.
There is a famous incident when President Thomas Jefferson (I believe) had ordered all the Indians from a certain region to be relocated to Oklahoma (or some such thing… sorry I don’t remember the details). Someone did take the case all the way to the Supreme Court, and the Supreme Court ruled that this was unconstitutional. Jefferson did it anyways, saying that if the Supreme Court wanted the Indians back they could move them. Since the Supreme Court has no army, it did not. In other words, the “protection of the constitution” is only as good as the governments willingness to follow it.
Fortunately for us, most people and most government officials are willing to live by and be governed by “the rule of law” which is why we have it so good. But that does not mean abuses cannot happen. They happen all the time.
OK, enough of my rant. This post will look much better in Great debates when a Mod moves it there.