That's the way the phone is supposed to work. Empty memory is wasted memory. Clearing it out would only make your phone slower as each app would have to fully re-load every time you open it.
Right now you can switch between a couple of apps quickly without having to totally re-load them into the RAM.
Imagine how annoying it would be to go safari-mail-notes-mail-safari-mail-safari if every single one of those switches required the app to completely re-load the content you were just looking at 10 seconds ago.