1. if they have to subpoena each site each time they need user data, it reduces mass surveillance risk. I'm okay with cops getting a warrant to access someone's gmail. I'm not okay requiring everyone to use email.gov.
2. I use a VPN and pseudonyms. they could unmask me if they cared to, but it'd be annoying. it'd be a lot more annoying if they wanted to unmask every VPN user all the time.