It's not. Out of US carriers, only Southwest and American fly 737 MAX planes, and they're not a significant part of either fleet.
I mentioned it because when I first heard that there were so many other regulatory bodies grounding the fleet, I was surprised about the FAA. When it was pointed out that many of those regulatory bodies represent countries having a handful of these planes to begin with, I was still surprised, but it made a little more sense that the FAA might have more to lose if the decision proved rash.
Not a trivial number for either fleet, but I imagine it won't significantly impact their business either.