Well, generally speaking in my eyes a remix is like seeing an idea with a nice foundation but having a vision for a different outcome.
For example I have remixed a few Darts maps because I saw problems with the originals so I remixed them to make them more appropriate for competitive games.
I've seen people remix Roper/Shopper maps with different colours, and slight variations which are nice.
I've also remixed some music purely because I enjoy the melody but vision a different pace or different emotions throughout "the journey" of listening.
Remixing stuff should definitely 100% NOT be forbidden, in fact should be encouraged, it's a form of creativity/imagination and being creative and imaginative are of the most awesome things about life.
I love remixes as long as they are different in an obvious way, whether I personally enjoy the remix or not is irrelevant, I will always support remixes.
It is extremely lame the fact Pizza doesn't link the original maps though, it's that attitude that ruins the beauty of remixing, should always give credit where it's due.