There are some that think nobody should slide head first, especially power hitters, as unprotected hands are much more vulnerable than feet with shoes on (and spikes to keep players out of the way) and leg bones are less likely to break than arm bones. Knees and elbows are equally vulnerable.
I definitely think you should never slide head first into home or first, and second and third only when absolutely necessary. The only times I ever went head first was diving back to the bag on pick off throws or to avoid double plays.
I think Bowell was being harsh, but also think he can list a few of Harper injuries that could have been avoided, and this one would be on the list for many.
As for the media backlash, they brought it upon themselves. They hyped him for so long and now he's not meeting their lofty expectations, so that must be his fault. He's going to be a very good ball player, but he'll be elite when he grows up.