Also, if dark side users can't use normal kyber crystals, why was Anakin able to use his Blue sabre when he went all Dark Side in EP3?
It's not that they can't use an already built one, but the process of them building their own turns it red. Either because they forcibly acquire one from a planet like Iilum or that re-purpose one taken from someone else.
It actually makes Kylo's saber make a little more sense now. He was still very much out of balance and unsure when he made it, neither fully dark nor light. It's no wonder he made a hash of the process.