I think there’s a trade off between the Driver’s and the Constructor’s championships. Sacrificing a Perez/Red Bull 10th place to deprive Hamilton/Mercedes of a point is obviously net even for the WCC, but net positive for Max in the WDC, while Perez is too far off to be considered part of the fight for 1st. At this point, it’s the obvious thing to do.
As you say though, I doubt they would have found that worth it if he was in, say, 7th… probably not even 8th or 9th, as you’re giving away WCC points at that stage and run the risk of missing out on the Fastest Lap if there’s a bad pitstop or a yellow flag or something similar.
I think there is some degree of weighting towards the WDC as well; this seems to be the one that most people remember. It’s an exciting battle this year, with many dimensions to it!