Oh it certainly is an MB policy! They even have an official term for it.

They call them TRP or Theft Relevant Parts.

All dealers are forbidden to sell any Part catagorised TRP without proper documentation and having sight of the vehicle in question. ie on their premises.

Now if that isn't restrictive practice I don't know what is.

I can understand a need for a secure paper trail with keys but the rest are just excuses to restrict access to the independent.