All good points in this thread. There are many possibilities of the cause of knee pain. Individual biomechanics can be a significant factor, shoes/wear, form, previous injuries, health, weight, other things outside disc golf. I suspect most players that experience knee pain from disc golf have serious form flaws that could either reduce or eliminate that pain.
I have terrible knees from all kinds of previous sports related injuries(ACLs, MCLs, menisci) and terribly flat feet(bad biomechanics design). Not to mention my ankle injuries. I have knee pain daily, but usually not while playing(putting practice is the worst for me). Custom orthodics and shoes with a solid foot bed can do wonders. Improving form/posture also helps a ton.