PeerTube uses BitTorrent to be more resilient if a video goes viral (everyone who is watching the video shares the load). If someone is already investing the money to host a PeerTube instance, I think they wouldn't mind if people were to actually use it. Otherwise, what's the point? For example, the admin of TILvids often advertises videos from popular tech YouTubers, who mirror their content on that instance.
PS: PeerTube also introduced remote video transcoding in the last update, so now it should be even easier to distribute the load across the network.