Has nothing to do with Roku. Doc Martin is produced by Buffalo Pictures. They decide what season to release for free. AcornTV owns the streaming rights to Doc Martin. Usually it seasons older than a year. So you were able to watch season 1-6 for free cause AcornTV and Buffalo Pictures allowed companies such as Roku, PlutoTV and Crackle to air those seasons for free. Same goes with just about any show. It is up to the network who owns it to release which seasons people can watch for free. Not everything you can watch on Roku, Firestick or Chromecast is free. If you want to watch Yellowstone you can watch season 1 episode 1 through Peacock for free. After that you need to pay.