I think disabled non-root access for security reasons. Which is understandable I suppose, since a malicious program sending screenshots could be bad. It easily allows unauthorized remote access and control of the phone if you let a program do it.
But then, this would be a case of trying to protect the user from themselves. Screenshots have some legitimate uses such as showing off (homescreens), troubleshooting, remote access (complete control of the phone from your PC), and stuff like that and sort of hinders some abilities of the phone.