The law doesn't care about competitive advantage. It cares about making sure that your competitive dominance doesn't translate into dominating other markets.
For instance, Microsoft dominating personal OS market and making sure that their product stays on top is OK. Microsoft using their domination of that market to drive competition out of other markets is not.
Again, I don't want to come off as needlessly argumentative, I'm just trying to get a better understanding of this topic.
The whole idea is that if you're going to let people build on your platform but compete with them and leverage your platform in order to do so, you effectively lured people into thinking that it was possible to build on your platform when in fact it was not, and that you benefit from this misunderstanding. And that's dishonest. If you open a "public market", then you should be accountable for keeping it fair and unbiased.
Keep in mind that the last part about how it "should" be is an opinion and not fact on the law.