Why? It's initial launch, it's not surprising plus this is a high end device at a very low price (for Nokia, hey this is far more powerful than N97, but cheaper than N97 initial launching price).

Beside, they have announced earlier for booking, so I won't say 300 unit is a lot, because this phone is already on the AP market for some time, many people might be looking/waiting for it.

The Motorola Milestone also targets the same 300 units pre-order, which also comes with FREE GIFT. The price is close to the N900 official set, but the spec is lower than N900. N900 have more hardware and it comes with 32GB internal storage.
Check out the price difference for other phone which have 8GB <-> 16GB <-> 32GB options. Sincerely speaking, the N900 price is really very competitive and considered low if compare with other competitors in the market.