My guess is that they install just enough memory to run the OS and some apps, with a bit left over for a few pics etc...which will run the device, but which will not allow full utilization of the capabilities.
It's really about making you buy a micro SD card, that's all. Many cameras and phones ship with a 1GB or 2GB card, but even those are not enough.
If you want your phone to be a laptop replacement/music player/video player/gaming platform, a 4GB or 8GB SD card is needed.