Scar doesn't realize he is facehugged or he just doesn't care.
The Queen doesn't care about killing an infected host (probably last of her kind surviving on Earth?).
A stab through the chest with a massive tail doesn't kill the Predalien chestburster
The Predators with "Chestburster vision" don't realize Scar is infected or just don't care.
To be fair, these are AvP's not Requiem's problems
1. What he was supposed to do ? Commit Predator seppuku ? By the time other Preds arrived he was dead already and couldn't tell them
2. Queen was really pissed off
3. Well, yeah, that's hard to argue. Maybe she missed it
4 These Preds weren't wearing their masks