So, a foreign carrier operating in the US doesn't have to abide by US laws when in the US (if they apply in this case)?
By that logic, why doesn't UA strip out the trademark symbols because they fly to Japan?
Not a question as easily answered as you may think. It's all about who pays attention to what.
Consider that nobody thought it was significant that Marriott referred to Taiwan under a list of "countries" where it does business until China made it an issue. Same thing here. Perhaps some distributor raised it as an issue to one US carrier and they all then agreed to do it to keep the peace.
Bear in mind that there is a mutual benefit and distributors either provide or provide at a discount in order to have their product served because it's a good way to expand its market.