If the FTC determines that the joint venture would form a monopoly, they could in fact block it on antitrust grounds. The FTC blocks mergers all the time.
Explicit cooperation in restraint of trade is always illegal, though.
Explicit cooperation in restraint of trade is always illegal, though.
I don't know anything about the collusion arguments though.
The difference is that explicit collusion is ALWAYS illegal. Otherwise innocuous activities that may have an anticompetitive effect (e.g. forming a joint venture) are sometimes legal, depending on what the FTC thinks.
That said, while their market share is certainly larger than 10/250,000, it probably doesn't even approach 10%. So you're probably right.