That's what I'm used to as well, but in my experience a lot of programmers take fold and reduce to be synonyms. A monoidal reduce is much less "scary" than a general fold. I suspect most programmers have never[1] heard the word monoid, let alone know what it means, and having to remember the meaning of a weird new word is enough to make most people dislike something compared to the simpler more familiar operations.

[1]Or if they have, their only encounter with it is the "a monad is just a monoid in the category of endofunctors" meme.

If the contraint is not in the signature, and cannot trigger a test failure with typical implementation, it doesn't exist.