The rule was probably intended to block public non-SSL pages, so your clipboard data doesn't ever get sent over the wire unencrypted.

Why does it block local pages? Well what benefit of is to Apple or Google if it were easier to make good localhost webapps?

Try deleting Safari site data(indexed DB etc) for your localhost site. You won't be able to. Hell, even deleting data for a specific public site is hilariously painful. Try adding a certificate authority to your iPhone. Try playing locally cached audio blobs via web APIs on an iPhone. There's probably 1000 more.