This is a bit pedantic, but I think you're misreading what's in the scope of the modal operator in the sentence. Saying "Under the federal rules, passengers have to be compensated up to $2,400 if they were denied boarding" means the following is necessary: that they be compensated up to $2400 depending on the details of the situation (e.g. when did they ultimately arrive at their destination?). That doesn't entail they must not be compensated more than that, which is how you are reading it.

That said, the link to the legislation is appreciated.