If they claim that the "undocumented" aliens need to drive, then we'll give them driver's licenses -- just Mexican licenses, not American licenses. If they claim safety on the roads, then we'll say fine, take our road tests and we'll give you Mexican licenses, not American ones.
There is no other reason for a driver's license except for the need to drive. A valid Mexican license will serve that need adequately.
-PJ
It's legal for them to use Mexican drivers licenses --- but that's not what they want --- crossing the border at the official entry ports only requires a US drivers license as proof of legal US residency.