They are forbidden because spriting is not an art form: it is merely the alteration of an existing piece of work in a small way. The content itself is small and often trivial, meaning that even the largest of collections is not suitable for its own thread. The megathread has been created for this purpose.
Spriting threads are acceptable only if its content is fully scratched -- that is, created entirely from scratch -- or if it is part of a larger project (like Amethyst). For these reasons, I suspect the answer to your second question is 'no'.