They are all suppossed to swap, and in the Japanese Series different heads "grant" different powers. Plus, as it's a part to be lost, the replacement Japanese exlusive heads need to fit all the figures too....

Same reason for the Powermaster engines, and the spare ones that were available for them as well.

Luckily they actually made another headmaster for Fort Max.... could you picture him with Spike and just an extra helmit? *shudders*