Actually, you are technicallly violating your contract with US if you have a restricted round-trip ticket and use it as a one-way. You could be sent a debit memo (or more likely your TA could) for not using all of your coupons.

However, the chances of this happening are extremely tiny unless you routinely make a habit of doing this. Throw-away ticketing very seldom gets one in trouble with the airline without a lengthy pattern of use and abuse.

You would not be violating any laws by checking in and not traveling.