Mail app should offer push yahoo mail server

I think the answer to the first question would be that Apple may be paying Yahoo some kind of concession (or else Yahoo is volunteering it for the visibility of being on the platform) for the feature, and so it is only seen as worth it on the iPhone. The feature is available officially -- it always has been. You just have to pay for it.

Yahoo and Hotmail both have push e-mail on RIM -- and yet neither of them have push e-mail in general on clients in Windows or OS X (not for free, that is).

As for ypops, it seems that development on the Mac side has been dead for a year plus. So if you find a version compiled for PPC, it's probably even less likely to work than freepops, because it's out of date.

EDIT: I guess one more complexity of why Yahoo mail doesn't work for everyone is the issue that it does work for some people. The Freepops plugin downloaded mail today for me on my PPC Mac without problems.

Have you tried setting your Y! Mail (via the web) to use the "classic" interface style? This sometimes makes it easier on fetching utilities like freepops.