If the system required you to enter the FDE-password whenever you open up the screen then how would background-processes, like e.g. SMS-receiving, chat and such stuff work? They'd only be able to access the disk when you have the display open and that'd obviously make the whole thing unuseable as a smartphone in the first place.
I get your point, but I disagree on the part where you write that background operations need the disk or else they can't possibly work at all. Current smartphones are not designed to work without accessing the disk, that's true, but in theory 1GB of RAM is plenty for processes like polling a chat server or SMS to run entirely in it.