Microsoft offers to sell OEMs like Dell Windows at below the market price, provided they agree to a set of terms (such as buying one copy for every machine they ship, to cut down on piracy). If Dell doesn't like those terms, it's perfectly free not to sign that OEM deal, and simply pay the market price for its copies of Windows the same as you or I would. Since Dell isn't doing that and is instead complying with the agreement, presumably the money it saves by getting cheap copies of Windows more than makes up for the sales it expects to lose by not offering OS-free machines.
Offering companies a discount on your OS is known as "predatory pricing" and is a "monopolistic practice." Note that giving away your OS for free [Linux] is not predatory pricing, and refusing to sell copies of your OS to OEMs at all [Apple] is not a monopolistic practice. I hope this clears things up for you.