Exactly It seems counterintuitive to take a perfectly good working piston designed for that particular AEG and swap it for one that has to be made to fit.
With regards to the hi-cap mag, it makes sense. Because if he does have any RS mid-caps he will probably keep them as they are currently harder to get than the RS AEGs themselves at the moment.
You are right trouble is you can buy the whole range of RS type 56's directed from the factory, they even periodically do 30% off the shipping although you run the risk of import duties.