Most Likely Illegal
I think this practice is 100 percent illegal. There are laws against conversion, which is defined as taking something and converting it to your own use. There are also laws against computer hacking. I am willing to bet there are already lawyers cooking up class action lawsuits over this and Mediacom will probably settle by discontinuing the practice and giving bill credits to affected customers, admit no wrongdoing, and the plaintiff lawyers will walk out with one fat check. I also see lawsuits coming from website operators as well. Then there is possible litigation over privacy as well. Many internet connections are used to transmit confidential information such as HIPPA Sensitive (aka. protected patient health information), proprietary company secrets, financial data, client/consumer records, and many other bits of information that has to remain confidential for various reasons. One of the clinics I go to uses web based medical charts and a lot of the practitioners access the medical chart system from their home computers and that entails information protected by the federal health privacy laws known as the Health Insurance Portability Practices Act or HIPPA. I would not want my confidential and protected health information getting in the hands of advertisers. The fines for breaches of HIPPA sensitive information are steep to the tune of $15,000 per violation.