Actually, we have very few laws meting out criminal punishment for crimes committed abroad, much less laws applicable to persons before they became US citizens. OTOH, Germany has pretty stiff laws against war crimes, along with a no-jury, lower burden of proof system. It would be very difficult to successfully prosecute these guys here, anyway. Most of their victims are dead or very, very old. Even with the greatly lower burden for deportation, many of these cases have dragged on for years. The Demjanjuk case left a sour taste in a lot of lawyers' mouths, the identifications were so vague and questionable, he was finally extradited on a technicality - that he had lied on his admission papers. Even Germany couldn't prosecute him, so we gave him to Israel.
As the alleged culprits have died off, the division of the DOJ devoted solely to the tracking down of NAZIs has become more and more desperate. In their waning years, they have overreached, IMO, time and time again to maintain their funding and relevance. In their zeal, they came to resemble more and more what they detested. BTW, I'm pretty much beyond accusations of antisemitism, having had a Jewish law partner for over 30 years (might be a reason to turn anti-semite) and attended NYU, where I was one of about 10% who weren't Jewish.
I'm just pointing out that the situation is a helluva lot more complex than it seems on the surface, and there're very good reasons our government has made the bargains and tradeoffs on SS and the like. One ironic comeuppance has been the sinking of the dollar against the Euro, with its attendant effect of devaluing pensions being paid in dollar amounts. That's a whole 'nother story...