[Cthulhu_]

I for one believe a lot of services - also e.g. Twitter, Youtube, etc - don't do much about the bots because they earn revenue from it. Twitter doesn't care if an ad impression is from a bot, they will get paid for it anyway - until the advertisers decide to pull out of course. I'm sure they are constantly trying to walk the line between too many and not enough bot activity.

I mean with email, spambots are always a net negative because the services that pass e-mails through do not get paid for them. But Twitter does. If you get a bot tweet on your feed, or a bot-promoted one, you engage with it and Twitter gets paid (in 'engagement', the magical currency).