If you accept any kind of government tax deferral, you are giving them custody and control of your account until all taxes are paid, anyway. It's a short step for them to just, sort of, take it - and give you an IOU in its place.
My 401(k) is in company stock. How would the government "take" my money without forcing a divesture of my stocks?
Wouldn't seizing my 401(k) be a de facto nationalization of the company whose stock I hold?
-PJ