mearls said:
It's a unique ability for that bugbear. It isn't part of the core grapple rules for a couple reasons. The big one is that it would be really annoying if the PCs (or a big monster) could do that in every fight.
I just really, really, cannot wrap my mind around how this is justified in-game. Bugbears are just big, strong, humanoids; if a STR 16 Bugbear can do something, I can't see any logical reason why a human fighter of similar size and strength can't learn the same trick. I appreciate your game design reasons, but I think the gain in simplicity is not balanced by the loss in verisimilitude.
As noted in the death&dying article, "believable" isn't the same as "realistic", and I well understand that, but having mundane, non-magical actions be available to one species of humanoid but not any other is neither realistic NOR believable. If the problem is "It's such a good move, everyone will do it!", then the solution is to make it less of a good move -- note that while such things can be done "in reality", they aren't the first choice of every hand-to-hand fighter in every combat, which means a game can model ways to make it difficult, but not impossible, balancing the utility with risk and difficulty. (i.e, it's hard to pull off (meaning you might waste a round trying instead of getting in a much more likely hit), or it exposes you to extra damage -- since D&D doesn't use facing, something like 'if the attacker makes the roll to NOT hit the human shield, he does +1d6 to you since you can't dodge very well', or whatever.)
Saying "DMs should handwave it" leads to what GURPS termed the 'flour in the face' problem -- if, in one combat, you let a PC get a big advantage by tossing a bag of flour in an enemy's face, they start carrying bags of flour instead of swords. Thus, the manuever needs to have risks/downsides or have a "works once, then everyone's heard of the trick and is wise to it" sort of thing built in.