Thing is that to get XP, and later 7 Starter, OEMs have to follow strict specs defined by MS. This then choked off any potential differences the OEMs could experiment with.
Yes, that was part of the deal. OEMs could choose to design whatever they liked, or choose to exploit a netbook/nettop design package that provided some discounts on Microsoft/Intel technologies.
OEMs were not obliged to market netbooks. They did it because they thought they could make money that way.
Similar restrictions came out of Intel, iirc.