If someone could explain, that'd be great. I'm presuming it's something to do with the fact that they're just 5ths with two of the same notes? Would like a good explanation! thanks!
All a capo does is change the pitch of open notes, it doesn't affect fretted notes - so as power chords generally don't contain open notes the capo has no effect.
Yes, this is why you can also play "cowboy chord" shapes nearly anywhere on the guitar with a capo in place.