Let us say for example a law to ban bitcoin was passed in the US, the price will go way higher simply because of the difficulty of buying it. Some people will still buy bitcoin even if it's declared illegal there.
+1 to this one, bitcoin's network doesn't depend hugely on USA, if they take this decision, this will be another proof of their dumbness which I don't think they are.
But still imagien USA banned bitcoin: what happens? Alternative ways, bunch of VPNs and mining/operation from current companies will continue from different location. In any way it's hard to run service from USA because of high costs, so this decision won't affect current US companies seriously, that's good. Then probably people will massively start to use VPN and access their coins this way.
If USA banks bitcoin there mustn't be massive panic to my mind.