What is the thought behind not giving credit for a player scoring on special teams? Half the leagues I'm in count the points and the other half don't. Seems to be rules overkill?

My guess is that it's because not every punt returner is available and/or desirable as a position player (i.e. DBs who are good returners), so by making KR/PR part of the DST, it equalizes the value of return TDs no matter who scores them. And if you go that route,
you don't want to give two scoring entities (DST and player) credit for the same TD.

You mean like when a Quarterback passes a touchdown to another player?
Look, it's so rare that a FF starter-worthy player gets a Punt or Kickoff Return TD, I say just count the Touchdown points to the individual AND the Defense/ST unit.
Stop making it too difficult.
How are you gonna sit there and tell the Reggie Bush owner that neither of those TDs against the Vikings got him any points because it goes to the owner of the Saints DEF/ST unit.
That's just silly.