Apparently, nothing. After I had narrowed the issue down to torrents, I called AT&T and confronted them with the evidence. On the phone with me they directly said "we do that to prevent illegal activity over our service lines."

No amount of "torrenting is not illegal" worked, and I didn't really expect it to. They don't care about the legality, they only use it as an excuse to keep customers from getting the full usage of the service they pay for.