I didn’t review Nitter source code but I assume Nitter caches Twitter posts because it helps stay low profile. If they don’t, that’s still a cost for Twitter because they don’t get to profile users or display ads, just waste bandwidth. They can brag about user counts but who would trust user counts from a bot infested platform?

It’s the same reason I buy Kindles. They’re heavily subsidised on the assumption that the end user will buy books from Amazon. If you buy a Kindle while discounted and never buy books from them you likely cost them money.