Apple considers the OS to be a intrinsic component of the device you purchased. The feature-set of the device was fixed at the instant they released the original 2G iPod touch SKUs, and this includes the features available in OS 2.x. OS 3.x bundles additional features that were not available when the 2G iPod touch SKUs were created, so they are not considered to be part of the deliverables of a 2G iPod touch purchase.
By purchasing OS 3.x, you are paying for those additional features that were not included with the original device.
The additional features delivered by OS 3.x were included in the baseline late 2009 iPod touch SKUs, and so OS 3.x is delivered by default in those iPods. All original 2G iPod touches still in the distribution channels at the time these new iPods came out, retained their original 2G SKUs and feature sets, and thus their original OS 2.x.
Because of situations such as yours, I think it is very unfortunate that some existing apps which had already been certified for OS 2.x have been withdrawn from distribution, and that new apps which could theoretically have run just fine under OS 2.x's feature set are now having their distribution limited to devices running OS 3.x.
For a very brief time back when OS 1.x was still current, Apple did experiment with delivering updated OSes which included new features for free, but they locked out access to the unpaid-for features until a separate purchase was made. This proved to be even more confusing to the average customer than their current policy of requiring fees for all major OS upgrades.
