SSI is forced retirement savings program. You get out an amount proportional to what you pay in. It makes sense from that perspective to have a cap on the amount of income taxed, because they have cap on the benefits you can receive.
Except that it is a was originally designed as a widows benefit.
It is not about getting what you put in, it is about a safety net for old poor people who have no income, whether or not you agree that we should do that, to me it is clear that is what the goal of SSI clearly is.