why cant we earn keyfragments by playing free champion?

hi all, so i pretty understand why we cant earn boxes , but i don't understand why we cant earn key fragments. so i have like 5 chests but if i wanna open them but i need to play only champions that i own for the key fragments.

i cant play free champions because i want to open those chests. so riot can you pleas change that so we can get key fragments by playing free champions other wise amumu will cry . have a nice day. sry for bad engilsh