The real goal is to make sure that people don't send bulk e-mail unless they can be held accountable for abuse of it. And almost all these TOS include an agreement not to abuse bulk E-mail in one form or another.
Contracts are just one way to hold users of bulk mail accountable for abuse.
For example, companies like Zero Knowledge sell anonymous net access, but delete accounts when they are abused. This puts a financial cost on abuse. If people who bulk mail can be held accountable if they abuse it, by mailing to people who don't know them and never requested the mail, a cost can be put on spam, helping to solve the problem. These free trial accounts often can be signed up for online, and the ISP has no way to enforce any TOS over free trial users, short of disconnecting them. However, after disconnection, they can just get another free trial, at that ISP or another one.