That's always boggled me.

Yeah, I never got that either... Maybe they want to show support for the artist and the piece they did, but don't want other people knowing what they like out of a sense of privacy? I dunno.

There's a setting controlling which favorites are visible (Site Settings -> Display Favorites). Depending on the piece in question, they may have it set to 'Hide Adult Only' or 'Hide Mature and Adult'. To check if they still have the work marked as a favorite, you'd have to look at the favorite list for the work. This can be found by clicking on the number of favorites for the work, either on the submission or in the stats page.

The 'big' reason (that I can think of) for hiding specific types of favorites would be for curating a public image. If someone has specific fetishes that they'd rather not proclaim to the world, they might hide some or all of their favorites.

That makes sense I suppose. Thanks, AM!