Not familiar with any laws (or how to read them lawyer speak) so might a dumb question...but does the USD being legal tender in the US mean that other currencies are illegal? Or the other currencies can still be be used but just not legally obliged to be accepted?
From my limited understanding as well (so nothing dumb about it, my friend) a business can not accept a foriegn currency. It can only deal in US legal tender. It's not illegal to possess them, but to spend them in the US, on US goods and services is a clear violation of US policy.
So by that thinking, and as one stated earlier, owning Bitcoins is not illegal. Using them is or at least technically.
Every single gas station and grocery store in the USA along the Canadian border accept Canadian dollars.