There's a difference between viewing their public pages and listening to private conversations, so your argument doesn't hold water.
If ISIS had a Verizon phone number that they publicized, I don't think it'd be stepping over a line for Verizon to shut that down. But if it was just a normal phone line, not advertised to the public, then Verizon has no way of knowing what it's used for.
Likewise if ISIS had an email address kill_them_all@gmail.com and advertised that this was the ISIS email address, yep. It's time for Google to revoke that. But if it's just disc_golf_playa_69@gmail.com and no one knows it's ISIS, then no one knows it's ISIS.
Breitbart isn't hiding its hatred, and it isn't hiding its store. Shopify doesn't have to infringe on anyone's privacy to realize they're running a store that lets people profit from hatred and racism.
There's no way to ban someone from the Internet. But they can be banned from various corporate sites if the corporation wants nothing to do with them. Which is what has been happening.