The way it works now is as follows: * Monopolies by themselves are OK * Abusing monopoly power in one market to leverage an unfair advantage in another market

The way it works now is as follows: * Monopolies by themselves are OK * Abusing monopoly power in one market to leverage an unfair advantage in another market is NOT OK. In the case of the latter there ain't a tax (the irony is that large behemoths are already avoiding paying their fair tax). A tax lcould also morally legalize the behaviour (moral is important in antitrust behaviour look at the claws/lobby of tobacco firms). On the contrary, it is a fine by a government subjected by a court of law. Every government has to start this lawsuit themselves though. It takes a lot of effort, time, money. On top of the fine, action as a result of being found guilty can be demanded as well (see history of US phone companies).