Yes they can. In theory govt can do anything. We have kool-aid drinkers like DamienBlack & Scientician to thank for that. They already ban people giving away music and movies on the net. Fortunately (or unfortunately for said kool-aid drinkers) this theoretical power does not always translate into authority on the ground.
You have me to thank for the theory that government can do anything? I don't follow you, please explain.
Yes, copyright law exists, people ignore it by sharing copies of copyrighted material with one another. The government can make any laws that they like, and people are free in this country to ignore them. The risk of getting caught is fairly low as well as long as you're just trading movies and music back and forth, much like the risk of trading BTC for goods and services directly exposes you to a much lower risk footprint that does cashing out thousands of BTC for USD.
The state that you live in is free to create laws. You are free to ignore them. You accept the risk involved. How much risk are you comfortable with? Its not "theoretical power" when the IRS starts suing people for tax evasion, or locking people up. That I believe that this can, will and does happen doesn't make me a "kool-aid drinker", it makes me a realist. When the feds go after BTC, you will have to recaluclate your acceptable risk threshold, again.
If the state is powerless to inhibit your personal liberty, there wouldn't be any need for Bitcoin in the first place. Your argument (was there one in there somewhere?) is a fallacy in terms.