Unlimited data plan for your phone, not a computer. That's not a breach of contract or any type of lawsuit. You're completely incorrect.
Don't get me wrong, I disagree with AT&T and Verizon all together on this whole tethering separate charges thing. I've always felt it to be ridiculous especially when they overcharge the hell out of it.. But, they have every right to do it.
You paid for your phone, not a separate device or computer. Your phone has the right of passage onto their network, and to use their data plans. You don't own their networks just because you subscribe to them.
Using a computer or separate device is unauthorized access to their networks. No one can argue this in anyway shape or form. It's in the contracts and their ToS guaranteed. I'm no lawyer but I'm fairly certain a breach of something like that without them knowing about it for a significant amount of time and usage, it's you who can get sued. Not them.