Mostly because they are on power trips and they will not be reigned in because the typical american is a scared little girl and LIKES draconian laws that make them feel safe.
Ask 10 Americans if they will trade freedoms in the constitution for more safety and security, and 6 of them will gladly give up MOST freedoms for it in a heart beat.
Ban Guns, Ban Soda, Ban Fatty foods, Ban criminals, regular neighborhood sweeps for your protection, checkpoints, etc.. Most americans are for them if they are attached to a promise of less crime and more safety.
OMG! If I let the cops set up checkpoints all over the city and I have to prove that I belong where I am and it eliminates the possibility of my house being robbed? WHERE DO I SIGN UP!!! OMG !! OMG!!
This is america, Moo...