>It costs the government money to put people in prison; not only the cost of their food, shelter, supervision, etc.

Except when being used as a talking point to stir a base, they do not care. If anything, their budgeting practice of use it or lose it creates a desire to spend as much as they can.

And as the other commenters have pointed out, it is very very beneficial for the most virtuous form of protest to be 'break the law and accept punishment'.