If they had monk levels, they would threaten those squares even while holding a crossbow. The interesting point now is: Could they grapple in that case? I think yes, because they threaten with their unarmed strike, and grapple is an unarmed attack.
How else can we explain the following sentence from the FAQ
> You can also disarm, grapple, or trip as a melee attack (or attack of opportunity).
Of course one could argue that there is a difference between attacking with feet and elbows and doing a grapple (where hands *maybe* must be free. Is that written somewhere?). Which brings up the original question: Could the monk drop the crossbow as a free action to do the grapple?