[clowd]

The pragmatic answer is: because the law says so.

The technical answer is: Verizon can't hear what you're saying to your mom, so they don't know/care the content.

The social answer is: ultimately there's no real difference between the two. If you use your Verizon phone to make harassing calls to your mom and she reports those calls to Verizon, or if you do something else that violates your agreement with Verizon, they too have the right to terminate your service:

"We can, without notice, limit, suspend or end your Service or any agreement with you for any good cause, including, but not limited to: (1) if you: (a) breach this agreement or violate our prohibited usage policies..."

Now, because you pay Verizon, and Verizon likes money, they may be more inclined to tolerate certain behavior as long as you pay your bill on time. That's their choice, though.