Unlike the Dora, the Tempest and Me 262 would cause a rather large game unbalance if unperked.
They dont unbalance the game in the usual sense - if they are free then everyone can fly them and no one has the advantage. The perk system actually breaks this symmetry because not all players can afford them.
What unperking them would do, and we learned this from the CHog, is to force everyone to fly them. So free planes sometimes end up limiting plane choice - not by preventing players from choosing a less advanced plane, but by strongly discoraging them.
What is good for the chog and tempest can be good for the La7 and others, at a much reduced cost.