After years of puzzled curiosity, I think I've finally figured it out:
If the music sells well, it's Rock.
This must be why so many classic Monk and Miles albums are labeled as "Rock" in digital music services' catalogs.
If the music requires at least half a brain to play, it's Jazz (or a hybrid of it).
This must be why a band like Behold...The Arctopus, in whose music one detects scarcely a hint of jazz, gets labeled as "Jazz-Metal" anyway.
In light of this realization, frequent MFEDI readers should be advised of an important change in editorial policy. Effective immediately, terms such as "Jazz-Rock Fusion" will no longer refer to the groove-driven, increasingly amplified stylistic mash-up pioneered by many prominent American and European instrumentalists of the late 1960's and early 1970's, but rather to a tiny group of works which meet the criteria for both styles as laid out above (that is, to "Kind of Blue" and Gorecki's Third Symphony). We apologize for any confusion.