Esperanza mentioned the magic words about taking work away from a local Mexican. It doesn't matter if you are doing it out of the goodness of your heart or in exchange for hamburgers or whatever. It still counts. The local security camera installer has one less job for every charity job you do, and he needs the work. And he is entitled to it, unless you have the legal right to be doing it. It's really pretty simple.
Couldn't have said it better myself. And it doesn't matter if you have 10 businesses in the States and own property in several countries, working without an FM3 (or 2) is illegal. Punto final! That said, I can imagine that spending 8 days in a Mexican jail is no picnic. I hope your lawyer gets everything straightened out for you in the near future.
Not only does one need an FM-3 or an FM-2, it's also necessary to have valid permission to work--the visa that BajaGringo mentions in Post #6 of this thread.
You absolutely can't work in Mexico
by simply having an FM-3 or an FM-2.
Sorry for my inaccurate post, esperanza. I was thinking of my own situation (I have an FM3 that allows me to teach English), forgetting that most MexConnect posters have FM3s (and FM2s) that only allow them to live in Mexico.