Imagine your bank, alphabank has 80% of personal bank accounts and 70% of auto insurance. When you check your transactions, you see a notice: "you could save up to x% by switching to alphacar."

That's the premise behind antitrust (even though in practice, its 50 years behind the times). If you have dominant market share, things that are otherwise lawful^ aren't anymore.

^Antitrust stuff like anti-competitive behaviour isn't criminal regardless.