Most spam is an entry point to some kind of either scam business or business business, not this just-for-fun semi-colon spam on bitchat. The ROI is pretty high. If you look at your spam inbox in gmail or whatever, that's what most of it is. These spammers will pay a lot to third party services to get into your proper gmail inbox, and sometimes that works. On Farcaster, the $7 fee to create an account didn't stop the spam at all, in fact it accelerated it. Spammers were happy to pay that $7 to access a cohort of people with money on a network with native payments. So Farcaster had to deploy centralised spam-mitigation tools on top, which they can do since they control the one big client. There's also just the advertising aspect to it, spamming becomes a pay per impression play just like any ad network. Or spammers taking money from people that don't like you to bring you down. Nostr hasn't seem much professional spamming yet because the impression pool is way too small, just a little phishing here and there.