They likely streamed from some other Plex server in the past, and that’s why they’re getting the email. The email specifically states that if the server owner has a plex pass, you don’t need one.

I got the email earlier today and it couldn’t be clearer:

As a server owner, if you elect to upgrade to a Plex Pass, anyone with access to your server can continue streaming your server content remotely as part of your subscription benefits.