The deals are structured to avoid this. They are generally structured as marketing assistance instead of competition or discounts. So rather than charging less MS gives them a big pile of money back under the terms that they use it for marketing or they don't give the money back but by advertising for the company on their own dime.
Under most of the worlds laws the only way to prove this as anti-competative would be real documentation of what it's really for like explicit emails indicating it's only for not making Linux laptops. Knowing that, the companies explicitly avoid putting that in writing. The result is everyone knows it's intent, but there wouldn't be a way to prosecute it without someone messing up royally and putting it in writing.
The only situation I've ever seen it prosecuted is where AMD spent the time and money to pursue the civil case where preponderance of the evidence rules instead of beyond a reasonable doubt. The government then used the civil trial evidence to basically force Intel to tack another couple items onto the civil settlement and give the agreement legal force.