Most of the time that's because when a puck goes inside a players equipment they actually make an effort to find it.
And that's hardly a dangerous demand to expect him to do something other than sit there. If anything the Kings were being pretty nice about not ramming their sticks in there like players normally would.
The bottom line is if he had made any effort to do anything but sit on the puck, it very likely wouldn't have been a penalty.
Seabrook knew exactly where the puck was, and did nothing. That's the problem. It shouldn't matter if the player intentionally or accidentally fell on the puck or not.
Basically it comes down to what the player "should" do in the interest of game flow, and what is
obligated to do according to the rulebook.
In that situation, Seabrook is not required to do anything other than sit there and wait for a whistle. He hasn't intentionally covered the puck, and he has no compelling reason to give the other team a chance to score from the slot while he's out of the play. So it's perfectly rational and legal for him to just stay still and protect himself. The ref can yell at him and tell him to move the puck, but the ref doesn't have a basis to penalize him for remaining in place.
The fact that the ref refused to blow the play dead put Seabrook in a very awkward position. It really doesn't look like he knew where the puck was at first, and he couldn't have stood up in a way that would have been both safe and smart. So he resorted to grabbing at the puck, which was indeed an illegal play.
What makes the call so icky is that by the book Seabrook didn't
have to grab at the puck, but the ref's insistence that he do
something to get the puck moving caused him to make that decision. That doesn't seem like a particularly fair or safe way to manage the situation. IMO it's a case of poor judgment by the official, though not technically an incorrect penalty call. I would prefer to see an annoying whistle for a faceoff in that situation, rather than an annoying penalty that changed the outcome of the game.