Grinching Up Mexico

I've been hearing and reading about gunmen going into schools and demanding the teacher's Christmas bonuses (known as aguinaldos) in the Laguna for weeks, and now it seems the same is happening in Monterrey and Hidalgo.

What doesn't make sense to me is why they go after schools. The Christmas bonus is mandated by law in every industry, so there's no reason why schools specifically would be more susceptible to this kind of activity. Indeed, since there is more cash at a business, and a group of mid-level factory managers would have larger salaries and therefore a larger bonus than a group of teachers, it seems like perpetrating this sort of activity in schools is not the best use of the assailants' energies. (Of course I am speaking from a hypothetical criminal's profit-maximizing point of view; robbing a bunch of teachers of all their Christmas cash strikes me as a pretty terrible use of energy.)