Yeah thats true. But frankly I never got why practically "everybody" takes their screenshots on a mac - It's almost like I've to go buy a mac just to take the screenshots and do my screencasts.
The developers may actually be using Linux/Windows, but the designers have quite a bit of influence if they control the final presentation of the product to the user. The bias towards Mac screenshots could be due to designers typically using Macs (a stereotype that has proven to be true more often than not from my experience).
In general, I agree that Helvetica looks much prettier on a Mac than on Windows (one of the main reasons I despise seeing font-family:Helvetica,Arial,sans-serif; on pages). Some other fonts don't look too bad on Windows, though.
I don't know much about font rendering; this is just what I've noticed. For what it's worth, some screenshots of the same text with font-family set to Helvetica [0], Myriad Pro, and Segoe UI on Windows 7, Firefox.
P.s: I did buy a mac last year.