I have a rMBP as well. You can use a little program called Retinizer[1] to convert YouView to retina compatible app. Retinizer works on almost any cocoa app, I have tried it on a bunch. All it does is set a flag in the info.plist file that tells OSX that the app is HiDPI compatible, the OS does the rest. You should also get Maximizer[2] to add full screen app option for it.
Ha! You are right. I usually watch videos in 720p, because my connection isn't all that great to YouTube servers, so I never noticed. Just tried it with a 1080p clip, and sure enough, the content isn't running in retina resolution. I guess if I haven't noticed after this long, I am not switching, but good catch.
[1] http://retinizer.mikelpr.com/ [2] http://chpwn.com/apps/maximizer.html