umm this might be a stupid question but if you get your CD key banned can you still play single player and it will just not allow you to play on battlenet or can you not play the game at all?
Yes it would be illegal to prevent a customer from using a purchased product without the customer signing or accepting an agreement.
But it is not illegal to prevent someone from playing on Battle.net because you must accept the End User Agreement to play, and they state that you will be banned from Battle.net if you are caught doing anything against the agreement.
Not like they can ban you from b.net either. You can untag/mute yourself easy, and if banned, you can get a new CD key just as easy. CD keys aren't hard to get.