It's not about being gentle with the carbides on the skids, it's about surviving training intact. Touchdown auto's are safer when done to pavement, it is far more forgiving of mistakes. On grass all it takes is a little left or right yaw on touchdown and an errant bit of turf to turn it into a rollover. But don't take my word for it, this question is easily researched on Google. Tim Tucker is as good an authority as you are likely to get, see his article
here, scroll down to the Grass vs. Hard Surface discussion.