…You can obviously have SOME matches that are mirror matches and SOME that are not. Not only that, but mirror matches themselves are the very thing that allow the matchmaker to control how often other nations see mirror matches or not.
If the matchmaker runs a bunch of literally-only-USA vs literally-only-USA mirror matches, then it can have arbitrarily few (even zero) mirror matches with USA that involve other nations. Or even any number of other-nation matches without USA at all. On either side. Mirror matches can “soak up” as many players as Gaijin wants, to free up other nations for [whatever at all else you can desire]. Without them, you’re almost just handcuffed and have way less control.
So pretty much the exact opposite.