You do have to be a monopoly in order for leveraging market share against competitors to be illegal.
You have a contradiction in terms - in a monopoly, the commodity is controlled by one entity. There are no competitors, there is no market share except 100%.
monopoly
Main Entry: monopoly
Pronunciation: \m-nä-p(-)l\
Function: noun
Inflected Form(s): plural monopolies
Etymology: Latin monopolium, from Greek monoplion, from mon- + plein to sell
Date: 1534
1 : exclusive ownership through legal privilege, command of supply, or concerted action
2 : exclusive possession or control
3 : a commodity controlled by one party
4 : one that has a monopoly
Thus, in a monopoly, there are no competitors. Antitrust lawsuits specifically deal with companies that do not have a monopoly. For example, see the cases vs Microsoft, where although MS did not have a monopoly on browsers, they did leverage their market share to unfairly crowd out competition and were sued for it.
An egghead is one who stands firmly on both feet, in mid-air, on both sides of an issue. -- Homer Ferguson