I believe this falls into a gray area for Apple that they don't want to mess with. You can not crossfade at all in the Music app on iOS. 3rd party iOS apps can't crossfade Apple Music (or will be pulled from the apps store). iTunes can crossfade but only Music you own now Apple Music or Music with DRM.
Spotify isn't completely immune (unless its changed from the last time I used it) as the crossfade option (along with other advanced settings) are disabled when the music is locally stored / cached. Spotify has integration into DJ tools however there are rules when using it, 1 being the songs can't be cached.
I can only assume Apple doesn't want to risk whatever liability Spotify is trying to avoid with their rules. I can't find statements from either Spotify or Apple on the subject directly explaining this. Regardless I wouldn't ever expect a cross fade feature for Apple Music.