Why does taking a screen shot require root access?
No idea why it wasn't available before, but as of 2.3.3 non-rooted screen shot support was added for system apps, at least. As eldarerathis noted, there are some articles out there implying that this would let any app capture a screen shot now, but according to an android engineer this permission is only granted to apps signed by the system image's keys - it is only available to be rolled into the OS, not as a feature 3rd party apps can access.