No one so far has gone the distance to get a legal decision.
Civil court cases are expensive; 90% of the time a cease and desist letter from a large company's lawyers is all that's needed to stop behavior the company doesn't like, whether they have a legal leg to stand on or not.
Facts aren't copyrightable; a classification system might
be, if it is deemed sufficiently creative by a judge. That's the rub, though: if it goes to court, the judge gets to make that call, and in order for the copyright to be upheld, Scott's numbering system has to be proven to involve a sufficient amount of creative input. A simple listing of stamp issues in chronological order (for example) would never qualify: that's simply a listing of facts, like a telephone directory, which is never copyrightable. Scott's numbering system is a little more than that, of course -- it often groups sets issued over periods of years together, and it splits air mail and other special-purpose stamps into their own categories. Is that enough? Maybe, but it's far from a slam dunk.
Amos Press can claim
anything it likes about the licensing of Scott numbers, but I suspect they don't want their numbering system to ever go before a judge, as there's a non-zero probability the judge will just throw their copyright out the window and they lose their most valuable IP overnight.