Disc golf and "Majors" have a weird history. You have events like Am World's, U.S. Amateur DGC and U.S. Masters that are classified as "Majors" but obviously don't have a field with the top players. Mid-Nationals is another dead "Major" with a weird format, Dave McCormack called it the "Sandbagger's World's" the year he won. U.S. Women's DGC is a legit Major for FPO, though.
Major doesn't have to be a classification for the top players only. It's supposed to be the very top tier events for each particular skill level/division, not specifically for pros or the open divisions.
The original grand slam in ball golf (from whom we adopted the "major" title) was comprised of the US Open, the British Open, the US Amateur, and the British Amateur. I don't see why disc golf having amateur or senior majors should be considered weird.