[ma2rten]

It can be illegal to use your market dominance in one area (search) to reinforce or create market dominance in other areas.

As far as I know it's only illegal when there is harm to consumers. Antitrust laws are meant to protect consumers, not competitors.

[WillPostForFood]

Consumers and competitor benefit are effectively intertwined. Consumers need healthy competitors to avoid being subject to monopolies, so antitrust laws protect competitors to benefit consumers.