I certainly can't think of any reason not to.

If they lead us into a trap they're worth more XP from the increased difficulty.

Technically ambushes aren't worth any more xp than anything else, unless there's an actual 'trap' to disarm involved.
That said, I would personally be inclined to give a proper increase for the difficult except for creatures that are *supposed* to be ambushing. Especially if players follow good roleplaying and only act on what their characters actually not.

Reminds me of a session that started with a player making a sense motivé and the GM telling him "He Seeeems trustworthy", in a way that was a bit too obvious. We laughed about it, then he narrated the whole session like this.
"there's PROBABLY nothing going on" "you don't Think she is hiding anything" "You miiiight be able to survive that" it was hilarious but also made everyone paranoid.