The fact that the payments are structured much like a pension is tangential. For example, nothing stops the feds from restructuring it into flat payments.
What do you base this statement on? The Federal Government enumerates the exact formula used to determine future social security benefit payments, even for people 40 years away from retirement. Given a known salary scale, a 22 year old can calculate their expected future social security benefit down to the penny. This future benefit is funded by a 12.4% tax/contribution from your salary. It's a % contribution for a defined benefit.
How is that any different than a state or municipal pension?
They are in fact so similar that, in some states, participation in the state's pension plan entirely replaces (rather than compliments) participation in social security.
There are other programs administered by the Social Security Administration (e.g. SSDI) that are welfare programs, but that's not what the previous poster was talking about.