Is it truly the only thing they could do? They could force phones to use different defaults. They could ban preinstallation of their apps. They could force preinstallation of competing apps. They could throttle them so their services get slow. They could force providers to show a large banner in addition to the site when showing it. They could I suppose add an additional tax on android phones to hurt Google, or even through the providers tax Facebook and Google usage until compliance. They could ban their apps and app stores. They could freeze bank accounts. They could send the army to destroy their hq. Banning their websites is merely the most sensible action in this situation.