I don't think its out the realms of possibility for any intelligence agency to set up a LOT of servers at different ISPs with different credit cards and account owners.
The problem with that approach is that it need to be done very quietly, slowly, and secretly, while being large scale.
If a few hundred thousands nodes or a few massive large nodes suddenly popped up, then the admins of the tor directory servers (or some security research) would start asking question. It wasn't that long time ago that a rather large cluster came into discussion because it looked suspicious, and the situation got resolved a few days later.
Then it need to say quiet since nodes require up-time in order to be weighted favorable compare to other nodes. During this time they will generate traffic, noise and like a few abuse letters. That mean the ISP will be in communication with the intelligence agency, which in turn either require lies which could fail or agreements which can leak.
Simply put, it is likely easier, more cost effective and less fragile tap the back bone ISP network and sort out tor chains when needed.
