I would guess that it may have something to do with safety concerns. UNSC weapons look a lot like real weapon (as Bungie found out when carrying a Sniper Rifle Prop out in public), and they may have decided that they didn't want to risk any kids getting shot for their brand.
It may also be the fact that it's an M-rated game, and squirt guns are children's toys. Imagine the backlash if a kid was killed imitating a game he wasn't supposed to play with a toy marketed to his age group.
English
-
There are so many water guns that look like real guns. Also bright orange or yellow is fine for distinguishing it's a toy. I mean kids are going to run around imagining it's a UNSC or Destiny gun anyways they should get out there and release fun toys. Not every kid is going to say I'm off to shoot a cop. It's pretending to be the hero from Destiny or Halo while it's a hundred degrees with water guns. Can't be afraid and worried about every bush containing a greedy lawyer or a blind policeman.