What premiums? The amount is withheld from wages, just as for non-Congressional contributors, and they aren't "premiums" that are withheld, they are "taxes". Social Security is funded by TAXES. Suggest a read of CCH's "Social Security Explained", which you can order from CCH Business Law Staff Publications at 1-800 TELL CCH. They're located in Chicago, if you live nearby.
We pay these amounts, including the employer's contribution, in the same way we pay their salaries and other expenses of government. This is no more a scam than paying the President or contributing to the retirement plan for Congresspersons in addition to SS. Or for heat and light in Federal buildings, or for judges or that courthouse down the street.
But you probably believe almost everything's a scam. SS has been one of the most effective programs run by the government and has brought older people out of impoverishment (maybe too far the other way, but even that's debatable), particularly workers who had illnesses such as black lung and disabilities acquired on the job. What AG said the other day has been said by many others (though not so close to national elections). The SS system needs repairs for many reasons and it will be fixed, one way or the other. And if fixed early enough, the changes won't have to be extreme.
If you work, you might get a pension and a 401-K, the opportunity to accumulate additional $$ in tax deferred vehicles like IRAs, SEPs, or Keoghs, and you cost ME $$. Even your company's contributions to your various plans are tax deductible for the company and increase the amount of taxes I must pay. If you are self-employed, a portion of your contributions are tax deductible. Because of that, my taxes need to be higher to accommodate your benefits. Why should you be permitted to accumulate tax-deferred dollars to be spent in the future? What a scam (just making a point).
The laws that govern all those plans are out there in writing. Some are good, some are less than that. Times change, needs change, public attitudes change, and demographics can change, and so the laws can be changed. But a "scam"? Don't think so.