If they are blocking access to channels that you pay for and authenticate specifically with your Internet provider, they are probably breaching their contract with the channel provider. But if the channel provider (such as History Channel) doesn't have an agreement with your ISP, there's nothing you can do about that. As you say, channels that don't require authentication are working, so they aren't specifically blocking a Roku device from the sounds of it. Since I have no such subscriptions, I can't say if it's something that happens. There's maybe one possible workaround, but we aren't allowed to discuss it on the forum.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.