"For purposes of the first limitation, relating to transitory
communications, “service provider” is defined in section 512(k)(1)(A) as “an entity
offering the transmission, routing, or providing of connections for digital online
communications, between or among points specified BY A USER, of MATERIAL OF THE USER'S CHOOSING, WITHOUT MODIFICATION to the content of the material as sent or received.”"
from https://www.copyright.gov/legislation/dmca.pdf
Notice the capitalized text. The user gets to choose not the provider.
We already know you can't discriminate your customers based on their race, creed, sexual orientation, gender, or age. So if they must allow everyone on, and they can't edit or modify the content a user posts, guess what. By banning people they are discriminating, by removing content that is not illegal, they are violating 512(k)(1)(A) of the Digital Millennium Copyright Act of 1998.