Why is an AM/FM radio receiver potentially more dangerous than laptop PCs,
gameboys, DVD players, and other electronic devices that are used quite
routinely on airplanes?

The other devices may have circuits that incidentally radiate a little noise
in the aircraft VHF band.
A broadcast FM receiver almost certainly has an oscillator running by
design, in the band.
Where it lands in the aircraft band, is determined by where it's tuned to.