If you're not just repeating an ancient talking point and actually don't understand this, it would be well worth spending some time on Wikipedia coming up to speed on the goals of antitrust enforcement.

The short version is that the actions of a non-monopolist are looked at very differently than the actions of a monopolist.

Additionally, you're dead wrong about "every Linux distribution", and even if you weren't, the fact that there are many of them would moot an antitrust argument.