My take is that it has to do with Mega compatibility (and future compatibility). On a Mega, the SPI pins and I2C pins are in a different place. The ICSP header, however, is in the same place on both boards thus allowing the Ethernet shield to work with both. By adding two pins to the digital header and calling them "SCL and SDA" future boards will have to route the I2C signals to these pins, no matter where they come from, once again improving shield compatibility.
The Gadget Shield
: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons