When netbooks were initially released, they were perceived to be a niche/hobbyist market, so putting Linux on a netbook made sense from both a fiscal and a market standpoint.
Microsoft realized that they were on the verge of losing out on a potentially lucrative market, so they quickly reversed course on sunsetting Windows XP, and under some very netbook-specific licensing conditions, made it available to manufacturers for cheap.
When the average user was presented with the choice of Linux -- a "new" OS to many people -- versus familiar XP which works exactly like their sons/daughters/job had trained them to use, and which runs all of their favorite apps, then it became pretty obvious which way the wind was going to blow.
I'm not saying that Linux shouldn't be an option -- I'm all for more choices in the market -- but there's really no conspiracy here, and no smoking gun.
(And yes, I know that you taught your great-great-grandmother to use Ubuntu in five minutes with no manual, and Wine sort-of runs most Windows apps with only some slowdown or glitches, and monkeying with your printer drivers for an hour is something everyone enjoys, etc.)