Exactly because more are buying it...

There is a lot more buzz around this device, so imagine what would happen to the future of their next maemo device if Nokia was known by everyone to have shafted all people who purchased the N900.

The next Maemo device would probably flop, or at the very least its sales be hindered.

I am sure they are expecting the Maemo6 device to have much greater success than the N900, being the 5th and final step.

It is in their interest to prove themselves on the N900.
(maybe this is why nokia was downplaying the N900, they were afraid of too much attention... but now they're stuck with it and they'll have to keep updating the device!)

Not many knew what an N800 or N810 or maemo was prior to the N900. The community was very small. It is now growing very fast and the N900 is barely out.