It is a question to ask. However, I’m not sure that they can tell the difference between legal and illegal downloads (somebody please tell me if they can).
If they can tell the difference, though, it would involve a lot of packet-sniffing, and by that I mean that they would have to nose into just about everything that people send or receive on the Internet. I’m not sure (even as a legal downloader) that I would want my provider that far into my business.
(For example, the provider would need to track my credit card use and look for proof that I had purchased the song. Then, they would have to look into who uploaded the song and see if they have a license. Most people, I believe, would be uncomfortable with their IP provider having such intimate details of their financial transactions. Plus, it’s a lot of work to keep up on all of that, should the Provider decide to do so, no doubt raising the price of their service and slowing the speed at the same time.)