Initially, these missions didn't give lower grade PERK materials. However, based on community feedback (and there was a lot of it), players were unhappy that they had to go down into lower tier missions to farm for Uncommon PERKUP.
This change allows higher level players to play missions of their level, collecting higher tiered rewards, while still able to maintain uncommon PERKUP.
However, the change that we made recently which gave back full investment of PERKUP has caused players to have a large amount of uncommon / rare PERKUP. I can bring this up with design and see where we stand after those changes.