These sites that could use Bitcoin to evade the financial embargo would likely break US money laundering laws. If not in the existing form then an easily modified one.

Also, eventually coins need to turn into real money to pay rent, buy food, pay for electricity. If your bank account is frozen that can be a big inconvenience to doing those things. There would have to be some kind of BitCoin for physical cash exchanges and you'd have to pay everything in cash. Not a very scalable solution for even medium sized operations.