Sig's program involves having an armorer give the gun a thorough once-over - I am not sure what this involves, but they must do some re-finishing and replacing the plastic grips at least. The resulting product goes into a red box and has a waranty.
HK wants you to buy a gun from them and keep it for life - I'm sure my USPs will live as long as me and I aim to get old - so there is a lifetime waranty on every USP, but only to the original purchaser.
Used USPs are a good bet, though.