WindowsXP cannot be bootlegged, because it requires "registration/activation" by Microsoft or it goes dead in 30 days. So your rationale is baseless in reality.
If this email is valid, then MS forcing Dell to ship PC's with an operating system is the equivalent of forcing Dell to not ship Linux (since most Linux users have their own copy).
The one thing that you do not understand is OEM contracting. If the price of Microsoft Windows XP is $350 retail, and Dell wants a cheaper OEM price model, then Microsoft can offer them $25 per license, but with conditions.
My business is a OEM, and we buy a lot of equipment under OEM contracting. Our purchase contracts are based upon quantity. 1-100 pieces costs $X, 101-999 costs $Y, etc... The more you buy, the better the price. This is standard business. But, penalizing Dell's pricing if they sell other OS's (or no OS's)? This is not normal in any industry that I've ever been in. This is a symptom of the supplier having too much power over the buyer. In a non-monopolist environment, the purchaser drives the terms of an OEM contract. BTW, I don't care what their contract terms are... This won't fly with the DOJ if/when this sees the light of day.
One other point... If this email is verified to be true, then Dell has no cajones.
there is no penalty for other OSes.