They haven't needed to, they can simply run exit nodes:
"If you actually look in to where these Tor nodes are hosted and how big they are, some of these nodes cost thousands of dollars each month just to host because they're using lots of bandwidth, they're heavy-duty servers and so on. Who would pay for this and be anonymous?"[1]
The only other plausible explanation I can think of, is that some Tor exit nodes are running on high-end machines without their owners' consent or knowledge.
Because no one owning, working or operating a ISP would ever, ever, dream of sponsoring some cable time to an anonymity network. They might be technical people who's job it is to work with computer networks, by why would they be interested in computer network technology? That would be like a technology company sponsoring open source project with work hours.
"If you actually look in to where these Tor nodes are hosted and how big they are, some of these nodes cost thousands of dollars each month just to host because they're using lots of bandwidth, they're heavy-duty servers and so on. Who would pay for this and be anonymous?"[1]
[1]http://en.wikipedia.org/wiki/Tor_(anonymity_network)#Exit_no...