Apple has a philosophy (which is very hypocritical at times, but hey) and that’s the quality standard they want to deliver. This is their argument for the AppStore, for example. And somehow they may also be right, the moment you can use the dedicated stores, Apple can’t guarantee the quality. †Daciru He says it more explicitly, but it comes back to the same thing.
It seems to me that this disaster was a natural consequence of that. Message “This site does not work well with your current browser, there may be problems.” So they will never work. In a way, it gives them a sense of confidence that they won’t let you work with something that might not be working properly.
In contrast, it is 10,000% rude that Firefox is not supported for a technically legal reason. This repeats the history of IE6 if your stuff only works on a specific non-standard feature set that isn’t standard, at least that’s an assumption, and I suspect it would be something like that (features are only available in chrome browsers), if it’s really something Firefox doesn’t do well As you pointed out, they could easily hide behind that excuse and thus put the ball in Mozilla’s court. Just being banned without explanation is sad and looks very unprofessional.
However, I can see where Apple’s (in this case, very misleading) position comes from.