The fact that Xenomorphs are handled differently from human adversaries by having special rules isn't arbitrary; it is a choice in design from the game creators and works very well
agree to disagree.
if xenomorphs can make aimed attacks theres no reason humans shouldnt be able to either. a human is just as capable of aiming their shotgun at someones head as a xenomorph is of aiming its inner jaws at someones head. there is nothing inherently special about xenomorphs that would allow them to aim for peoples heads but not humans. it is arbitrary.
I wouldnt always allow aiming at specific body parts. but certainly if you sneak up on someone and they fail their observation roll you should be able to aim your shot where you want (albeit with a hefty penalty of say -3 or -4 to your attack roll).
Also im not a big fan of signature attacks on Xenomorphs. Xenomorphs being speed 2 or speed 3 combined with having instakill attacks can cause the party to get completely killed off in one turn. Ive seen it happen. Its not a fun experience for anyone. I dont really like unconditional instant kill attacks (especially on speed 2-3 creatures). I believe the circumstances have to make sense for that type of attack to be setup. Which is why I dont use signature attacks and instead use my own flowchart. I only let Xenomorphs headbite targets that are surprised, grabbed, prone, broken, or otherwise immobile. That way the players get a fair chance of reacting to it, and it builds suspense because they know the headbite is 100% coming if they dont stop it.
My advise is dont let the rules stop you from telling the narrative that makes the most sense. And also dont make your players feel like they couldnt change their fates. Always give them a chance to get out of an instant kill attack with a roll. Whether its an observation roll, block roll, opposed CC roll to break being grabbed, etc... or their solution could even be something entirely unconventional that isnt covered in the rulebook that they should still be rewarded for.