I disagree. Because SS is merely another tax on earned income (contrary to popular belief, SS money collected is comingled with money from the general fund; there is no SS "trust fund"), those who are legally working in this country should pay SS taxes just as they pay any other taxes. If they don't like it, they can return to their own country to work. Paying SS tax is just the price to pay for the privilege of working in this country. We don't have to let them in.
I agree with you that they should pay. We already have the growing number of federal employees who are excepted from Social Security. Of course, the federal employees benefit from the confiscated Social Security taxes that the rest of us pay in.
Also, Americans don't have a high enough birthrate to keep this Ponzi scheme afloat. Mexican families have more children who can work and pay for aging baby boomers. I don't like Social Security, but since some of us have been paying forever and will never see the money, why not confiscate legal immigrants money as well? Maybe that will create another class of people interested in reform.