Given that they block login sharing, I think it's most likely the following scenario:
Friend with a Netflix account comes to your house, casts Netflix to your TV. No need for you to have a Netflix account in that case. Whereas, they want to force you to also have a Netflix account to watch on your TV.
Friend with a Netflix account comes to your house, casts Netflix to your TV. No need for you to have a Netflix account in that case. Whereas, they want to force you to also have a Netflix account to watch on your TV.