Nobody should have to endure getting fucked up the ass while holding their own legs in the air but hey, it happens. And as uncomfortable as this poor bottom may be during this pounding, we doubt he complained much afterwards.
Besides all that, we know there are way more uncomfortable positions to get stuck in while you're fucking somebody. We just think that, if you're fucking somebody while they're on their back with their legs sticking up in the air, you might as well hold one of them up. It's the considerate thing to do.