It is actually possible to enable hi-dpi mode (retina) on all Macs. If you try it out, you'll understand why Apple don't enable it though... You have to run at a quarter the full resolution in terms of working area, and for the non-retina displays, that's not all that usable. In case you want to try and turn on hi-dpi mode though:
Easy way:
1: download Xcode and Quartz debug
2: Open the Quartz debugger
3: Click "Window" in the menu bar, and pick "UI Resolutions".
4: There's a button that says: "Enable Hi-DPI mode" tick this
5: Now enter your password and System Preferences should have new resolution options for Hi-DPI mode.
Harder way:
1: Open Terminal
2: Type "sudo defaults write /library/Preferences/com.apple.windowserver.plist DisplayResolutionEnabled -bool true"
3: You will be prompted for your password and asked to either reboot or log out all users
Beware! Some users have reported problems using the terminal command. If anything goes wrong in the process, using the Terminal method, you will have to boot into single user mode to correct it.
Link to a guide for the Terminal method:
http://cocoamanifest.net/articles/2013/01/turn-on-hidpi-retina-mode-on-an-ordinary-mac.html
Link to a guide for the Quartz Debug method:
http://www.mactrast.com/2013/01/how-to-enable-retina-graphics-on-a-non-retina-mac/