Yeah, I was just thinking about this. The internet isn't really a web, like a spider web, it's more hierarchical, like a tree, where the major backbone infrastructure is the trunk and large branches, so to get from any leaf (node) to another, all paths go through the major branches. If you live in a small town that is an hour away from a major city that has an IX, all of your traffic goes through that IX, and thus all of your connections to US Tor entry points can be watched. If the destination is one of the major service providers near San Francisco, where there is also conveniently an IX (or maybe a few?), all proxies including Tor could be useless.