Source? In a country as lobbied and lawyered up as USA what kind of crazy antitrust laws can affect a mobile OS that is an absolute underdog with single digit market share and two HUGE competitors?

From what I gather (and I don't have any more source than word of mouth/rumor, either): at that point of Windows Phone it wasn't any laws or direct legal reasons, but the ghosts of the US DOJ and EU Court of Justice rulings that had Microsoft still worried about changing business models and giving anything Windows related away for free.

Though a lot of that was still momentum/inertia, too. Microsoft had always charged for Windows on every platform, that business model was largely sound and safe.

In Windows 10 there was enough push to change that business model, but the hesitancy in the Windows Phone 7 and Windows Phone 8 timeframes to do that probably did lead a lot of handset manufacturers to "free" Android SKUs.

Back then, it was said they earned more money per device from OEMs making Android through patents (e.g. FAT) than OEMs making Windows phones.