HTC pays Microsoft for using Android and soon Samsung may join too. Since Googles smart-ass bidding for Nortel patent did not play out taking over "free" OS may seem to expensive
for these companies: pay for development and pay patents fee.
But if they used their own OS, they would pay for development and patent fess, too. And if they use a commercial OS, the patent fees are just rolled into the final price (and other companies can still go after them for other patents; no software includes patent indemnity).