You are ignoring the possibility that he gave it a go to see if he could play through the injury, and once he realized he couldn't, he quit.
An interesting thing happens when Tiger WDs from a tourney. Lots of folks assume the injury worsens as a result of poor play, and completely ignore the possibility that poor play could be a result of a worsening injury.
Very good point.
If someone is injured, they have no responsibility to risk injury in order to continue playing. It's really not relevant that he pushed through a worse injury in the past. Maybe he learned something from that experience and has decided it's not worth risking it again.
The way I'd think about it is that he thought he might be able to finish and he gave them 11.25 holes on Sunday before he realized it wasn't going to work. I'm sure the fact that he was out of contention was a factor, but it's impossible to gauge how much of a factor.
And, let's not kid ourselves. Anyone with a possible injury considers the importance of the event when they decide how much to push it. In addition to the win itself, the broken-leg US Open win is a story that people will be talking about for decades. I think anyone who claims they'd treat the decision to withdraw when in contention in those circumstances exactly the same as they'd treat a possible 45th place in a smaller tournament is blowing smoke.
Plus, sheesh, it's not like he hit a dozen 7I into the water before walking off the course...