Do you have a source for this? As far as I can tell, the agreement would apply to illegal aliens once they move back to Mexico. From a story in the Washington Times in September:
Jo Anne B. Barnhart, commissioner of Social Security, said current law requires anyone receiving benefits from the United States be a legal resident of the country being lived in at the time the benefits are paid.[Emphasis added]
She also denied reports the administration is trying to change that and allow illegal immigrants in the United States to receive benefits.
"Any totalization agreement that would be signed with Mexico would not have anything to do with immigration," the administrator said.
But witnesses and lawmakers said the law still allows an illegal Mexican immigrant to earn credits in the United States then return to Mexico and receive benefits from the U.S. government, since he would then be living legally in his home nation.
This money is taken out of people's paychecks who worked here legally, with the understanding that it would be paid back, his phrasing seems to indicate that this is money that belongs to the government, and not to the very people that it is going back to.
The money you and I pay into Social Security doesn't belong to us. It funds the retirement of current retirees. We do this in the expectation that future generations will fund our retirement.
Does this "compact between generations" apply to noncitizens who work here for several years then leave? Assuming they were here legally, my belief is that we owe them something, but not necessarily full participation in Social Security. Just my opinion.