Banning them will invite antitrust regulation, because there's a conflict of interest between giving away the browser and crippling it to benefit their advertising business. I suspect that for the legal department, it's too close to the historical example of Microsoft leveraging its operating system dominance to benefit Internet Explorer and hurt competitors like Netscape. If they bury the anti-adblock strategy under layers of technicalities, they can plausibly deny that they're targeting particular vendors.