Uh huh. I'm just absolutely sure that they check the papers of the Mexicans they hire and turn away those who can't prove legality. 
They very well might. Most of the legal Mexican immigrants I know have a serious hate on for the illegals, people who went through the trouble to do the paperwork tend to find people who didn’t annoying.
Of course most restaurant people say that Mexicans have pretty much taken over the back of the house in the whole industry. Doesn’t matter what the ethnicity of the cuisine is it’s probably cooked by a Mexican. So really you shouldn’t focus your boycott on Mexican restaurants which are probably owned by legals, but ALL restaurants, if you can’t trust some of them then you shouldn’t trust any of them.