The reason ETFs are an antitrust concern - really I mean index funds here - is because of common ownership. For example, I think I read somewhere that something like 60% of United's stock is held in funds that also hold stock in other airlines. That's a massive conflict of interest because managers are required to act in the best interests of the shareholders. If United and Delta are majority-owned by the same group of people, that means collusion and market division. And that's exactly what you see in major airlines, telecoms, insurance, and many other industries in the US.
That's the antitrust theory, at least. In reality common ownership actually reduces collusion because, among other things, there is no real benefit to owning two companies that divide a market versus a single company that dominates it. The antitrust bit is really about inventing a way of undermining the legality of an investment vehicle that denies very rich financial services corporations funds management fees.