Um..lol Do a Google search and you will find lots of info on Steel cases are bad for guns.
I will say again.. Says who?
Sorry but "google" is not an acceptable answer. Please try again.
Here. I did google it and the top 5 results say you are WRONG WRONG WRONG WRONG WRONG.
Click this and see!
http://lmgtfy.com/?q=Steel+case+bad+for+AR-15