I wasn't arguing the letter of the rule, I'm just saying in the last four years of watching football that it seems to be the standard way to block and I've never seen it called. In fact, I've heard ESPN talk about it several times saying, "as long as you keep your hands inside you are ok, but if they slip to the shoulder you will draw a flag."
I don't doubt the rule book doesn't allow it, but it's how all the receivers are being trained to block and obviously the refs are, as a group, not calling it.