The turtle that they were based on doesn't have a screen, you have to program it via external means, and I think that would be a cool optional feature for ComputerCraft.
Based on a post I made a couple days ago
JJRcop, on 22 May 2013 - 04:02 PM, said:
Actually, let's imagine turtles without screens, where you would have to install all the programs via a disk drive, and where you would have to need wireless turtles to get the same amount of interactivity we have right now.