For the Road Warriors: Why are Burnouts in Drive a Bad Idea?

I just killed my 904 doing burnouts.
Put it in first and stabbed the throttle, smoked the tire until the entire parking lot at the shop was FULL of white smoke...

Pulled the car back in the shop and left. Next day went to drive the car and it would hardly back up, drove out the lot and RRRRRERRRE massive slipping in drive. Trans is DONE...now the trans is coming out today..
It may have just been it's time to go or maybe it was the new motor or God knows what but it's done and the last thing I did was a first gear burn out.

I'll post some pics later when I get it out and apart.