Apparently apps from the Mac App Store are wrapped in DRM (not surprising) and this DRM is somehow tied to the motherboard hardware, not the OS install or the disk drive hardware (more surprising).

I wouldn’t have realized this until I “upgraded” my Mac Pro by buying a newer one, and plopping the SSDs from the old one into the new one. This let me avoid installing or configuring the OS or apps; everything ran just fine but faster; everything, that is, except apps from the Mac App Store, which all (individually) prompted for my MAS credentials with a dialog about how “this app was purchased on another computer”.

(I expect this scenario and dialog are more common when copying the app bundles from one computer to another; that’s an obvious case of purchasing an app on one computer and running it on another; moving the whole drive and OS install is less obviously a new computer — hey, it’s still got the same hostname — but it triggers the same scenario.)