Griefing on PTR - Why?

I've never particularly understood why people take their 80s to starter zones and systematically slaughter vendors and questgivers on LIVE, and I am completely mystified as to why people are doing it on test.

If the purpose of PTR is to allow players to test out game changes, then what possible justification can there be for spending 30 minutes or more in Auberdine killing everything in sight while laughing?

Can people be banned from PTR for doing this? It makes actually TESTING an incredible pain in the butt.