But only exit nodes are the problem here. Traffic between nodes is encrypted anyway. If the encryption is sound (and there is no reason to assume the contrary), they may collect as much as they want.
There is anyway no guarantee at all, that non-TOR traffic doesn't cross borders. And you can't assume that any three letter agency acts within the (intended) legal boundaries. To be safe, only end-to-end encryption helps.
Sorry if I wasn't clear, but this is exactly my point. Most of your traffic as an American will stay within the country's borders because most of the services you access are in the US. By using TOR, your traffic will now appear to come from an exit node that has a greater than zero probability of being outside the US. The average American user thus has increased the likelihood of their data being analyzed by the NSA by using TOR. Under great-grandparent's particular threat model, the user is worse off.
Also, we can assume that the NSA operates within those bounds because that's what Snowden's leaked documents say in describing their systems. We have their internal documentation as proof.
My worry is that by using Tor at all you become a target for active monitoring, even if the content and destination of your Tor communication can't be decrypted.
Which is why it is important that many people use it. I don't think it is a viable strategy to MITM everyone (at least it will not go undetected), if that is what you assume being a target for active monitoring. Or to send agents to every house. If we are forcing them to do that, we have won.
There is anyway no guarantee at all, that non-TOR traffic doesn't cross borders. And you can't assume that any three letter agency acts within the (intended) legal boundaries. To be safe, only end-to-end encryption helps.