The device change limits are imposed as part of the content agreements with the film studios and content providers and are there to primarily prevent people sharing their logon details with friends and swapping devices willy-nilly.
You will find the exact same device change restrictions on Virgin's TV Anywhere and the BT TV app for the self same reasons.
You will find the exact same device change restrictions on Virgin's TV Anywhere and the BT TV app for the self same reasons.