Bots use VPNs for all sorts of BS. This quickly gets the VPN servers added to bad-bot blocklists. These blocklists then get shared around by various security services.

Or they’ll see a ton of connections coming from a single IP, which obviously throws up a red flag as a potential bot farm. Even if it’s all legit users, the fact that the single server has hundreds of simultaneous attempted connections tends to throw up a red flag. It’s a pretty obvious “this isn’t just a single user trying to reach the site normally” red flag.

Third, it could simply be about data collection. Reddit is notorious for this. They want that sweet sweet telemetry data to sell to ad companies, and a VPN stymies their ability to find your real location or track your usage across devices.