Are you seriously arguing that derailleurs aren't determined by slack and max rear cassette size? Or are you claiming that there even exists a Dahon or Tern 20" 3x8, 3x9, or 3x10 geared bike with a large slack (please give an example)? Or are you claiming that the bike with the provided gearset would have worked with a compact derailleur? (I dunno, seems unlikely).

There's no logical reason to complain about the derailleur, if it's a function of the gearset. Instead, ask why the bike is sold with that gearset, if you like. But first you might want to argue more conclusively that other bikes are always sold with your "common sense" derailleur clearance.