There wouldn't even need to be any explicit throttling - when a site gets slashdotted or whatever everyone trying to access it slows to a crawl, but it's not because individual user's ISPs are throttling them. An ISP that was saturating its outbound links might drop packets according to some algorithm to ensure "fairness" rather than at random, but that's still not throttling, that's "our network is too congested to keep up with demand".