In part, because historians & philosophers have generally castigated the Allies for not being more diligent in their denazification efforts. Morally, of course, they're right: people like Wenher von Braun were very likely guilty of crimes against humanity, and very possibly should have been hanged (he wasn't, of course, and as a result we got to the Moon). In my late-90s education this was all taught as almost unmitigatedly bad thing; the general belief was that we should have done the Right Thing, not the Pragmatic Thing.
Our experience in Iraq indicates that perhaps the Pragmatic Thing can also sometimes be the Right Thing. We might have been able to avoid a lot of injustice with a relatively small amount of injustice.
For this explanation to make sense, you need to consider "mass indiscriminate firing" to be "justice." Those vocal critics of things like Operation Paperclip could have told the Bush administration that was ridiculous in 2003. They were never asked, almost no one was. The occupation strategy was slapped together last minute by people chosen for their political ideology.
In a regime where membership of The Party is mandatory if you want a good job, it's reasonable to assume that many if not most members of The Party were merely ticking a box.
Then why didn't they clearly see the problems that happened when they tried this is Germany and Japan?