Well, there is. If you get caught you get to jail.

Whereas if you get "caught" saying this generated melody is your melody, nothing happens. It's a totally valid thing to copyright. (And even if it wasn't, unlike the store theft case, there's no way for anybody to tell and prove it's not yours anyway).

But in any case, you appear confused as to this.

Whatever tool you can use to create a melody, the melody is still yours to copyright (unless somebody else came with it first and copyrighted it). You don't have to "think" of the melody or write it on the piano, or something.

In fact tons of melodies nowadays are written partially or wholly by compositional tools.

People using those tools have the regular claim to the output melodies, regardless of whether the tool is some DAW utility or "random melody" button (all of which exist), a music synthesis algorithm with some params and seed, or even an exhaustive, minimally creative tool to iterate over all possible melodies.

(Actually some musicians/coders did exactly the latter: https://www.vice.com/en/article/wxepzw/musicians-algorithmic... )

There's no rule "you can't copyright a generated melody".