While violating a sites Terms of Service/TAC isn't against the law, doing so will give them the incentive to file a lawsuit against you. In the US you can sue anybody for anything - even if you don't have a case or plan to win. It's just kind of how the country works for some reason.
That being said, Perkins & Coie has a nice habit of sending cease and desist orders to people that don't properly align with federal guidelines on how C&D's should be structured. In your standard C&D letter you're supposed to ask the offenders to stop abusing X or you'll be facing a lawsuit. In the C&D's Perkins & Coie send out, they demand that you:
- Give them any money you've made while operating the services/campaigns they believe you've abused
- Give them access to your customers data (emails, passwords, addresses, contact information, order information, etc)
- Give them your intellectual property (source code, infrastructure, etc)
- Give them a list of all of your contacts who do similar things (this is a big no-no, but everybody always caves in at this point and it just contributes to the domino effect that they're looking for)
- Demand that you meet a list of outrageous criteria for eternity
This is not allowed. If you receive a C&D from anybody that asks for anything above, you can ignore it. All in all, if you do get a Cease & Desist letter from anyone, technically you don't have to do anything. You can ignore it and comply, respond and only comply with the basic terms they want met (ie: stop spamming them

), or ignore it entirely and risk a lawsuit if they're super-serious and think you're worth their time.
Source: I've recieved C&D's from many, many companies and I've never been sued. Consult with your attorney before even responding if you get one to figure out what is best for your current situation. I'm just a guy on the internet - not a lawyer.