ISPs also have Terms of Service and acceptible use policies. If a user violates those, the ISP certainly has the right to terminate the user account.
Example:
Comcast.net Terms Of Service - Acceptable Use Policy
Quote:
Conduct and information restrictions
undertake or accomplish any unlawful purpose. This includes, but is not limited to, posting, storing, transmitting or disseminating information, data or material which is libelous, obscene, unlawful, threatening or defamatory, or which infringes the intellectual property rights of any person or entity, or which in any way constitutes or encourages conduct that would constitute a criminal offense, or otherwise violate any local, state, federal, or non-U.S. law, order, or regulation;
post, store, send, transmit, or disseminate any information or material which a reasonable person could deem to be unlawful;
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Bold portion done by me. That paragraph would also apply to downloading/sharing child porn and other illegal activities.
When a customer agrees to the TOS and AUP, they have no reason to complain if, when they violate the terms and policies, their service is terminated or limited in some manner.