Why don't airlines let us use Airplane Mode/Game Mode on our portable electronics?

I mean, the whole purpose is so that we can use them without sending out wireless signals, right? That’s what Wiki says, anyway. So why, then, do pilots specify that Airplane Mode is not acceptable? Is it because FAA regulations haven’t caught up to technology, or do the signals still get out, or what?

Edit: I am talking about Southwest, but I thought I remembered United doing it too.