I'm not even saying that OpenAI/MSFT are being anticompetitive, but I certainly see no problem with the FTC keeping an eye on them, especially given Microsoft's track record.
"is the enforcement of civil (non-criminal) antitrust law and the promotion of consumer protection."
https://en.wikipedia.org/wiki/Federal_Trade_Commission
So it could be antitrust or consumer protection.
Microsoft is a monopoly in other spaces, and could be leveraging those monopolies and their partnership with OpenAI to monopolize parts of the AI space, which would be illegal even without having succeeded.
You do not need to be a monopoly to engage in anticompetitive practices that are illegal.
Monopolization begins to matter more with things like mergers. It is not necessary for other illegal anticompetitive practices, such as price fixing, for example.
Collusion is illegal - full stop.