Favoring your own products is fine when you don't have a market-dominant position. The rules change when you reach a certain level of dominance: that's the gist of antitrust law.
So it doesn't matter if you're doing the same thing you always did, or that other less-dominant companies do regularly. Certain pricing strategies, product-tie-ins, contractual agreements that were legal, stop being legal.
I'd agree that's kind of a mess because there's no bright-line indicator you've crossed the threshold. It has to be argued, via a bunch of somewhat subjective and ad hoc per industry standards, after the allegations of abusive behavior come up.
So perhaps there shouldn't be any antitrust law. Or it should operate on totally different grounds. But given the way it has been legislated and practiced, doing the same thing other rational companies do is not an accepted defense.