Who can blame them, the UI blows.
Since devs themselves also do not get any user-specific information when an in-app purchase is sold, it becomes pretty much impossible for your support channels to verify someone actually owns what they say they own.
Until the API improves (not holding my breath, it's been like this for, what, 3 years now?) IAP-driven upgrades will hurt both users and developers.
[0] http://itunes.apple.com/us/app/ascension-chronicle-godslayer...
This makes sense for low-price + IAP apps, but not the free + IAP model mentioned in the thread you're replying to.
Free + IAP seems fine, and it's probably been the most popular PC/Mac software sales method since the 90s.
Not a big deal for demo + full version apps, but when you pay full price for an app (or download a free, non-demo app) and keep getting pushed to buy other apps it gets pretty annoying.
Some apps have done it tastefully, like Temple Run. Others like Scramble (and my assumption is all Zynga apps) just make it a drag.