Yeah, hGH is a great PED in theory. That is, it affects cellular biochemistry in ways that strongly suggest that it should be potently anabolic with few if any side effects. But that's all based on in vitro studies and hGH has never been demonstrated to do jack in vivo. My somewhat educated guess guess is that cellular in vivo responsiveness to hGH is regulated even more tightly than in vivo production of hGH.

The incentives are entirely set - admitting use has no benefit.

The one case it does is for a player that actually did fail a test. Pettitte and A-rod get their positives completely overlooked. Manny Ramirez owned up to his first one and took comedy but not heat for it. If you fail and admit, there's nothing left for the writers to get sanctimonious about and punish, so they go find other meat.

Admitting wouldn't help Piazza or anyone else who's got nothing but bacne though, of course.

When did Pettitte test positive?

The problem was always the union considering this a private pee issue and not a workplace safety issue. They should have led the way in forging a drug policy because it was in their best interests. And if they had taken the lead, instead of seemingly getting dragged there, they likely would have fended off much of the witchhunt that followed. After all, the policy had nothing to do with the problems faced by mssrs. Bonds and Clemens.

Much of this speculation, because no one knows what would have really happened in the end. And another issue is how do you expect 20 and 30 year olds whose livelihood depends on watching video, grooving a swing, maintaining muscle memory, etc to suddenly immerse themselves in legal and sociological issues? It's not realistic, even though what you're saying does make sense to us looking in.
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In the case of Piazza a better word would be "maintains." As in "Piazza maintains that he did not do..."