* Offer to charge less usage if a duplicate block is detected, but allow users to pay full price for privately-salted storage. This is similar in principle to how Data Saver works on your mobile device (opt-in to allow a man-in-the-middle to compress or downsample your data)
a) Once you know the chunk size, you can determine based on pricing whether another customer has that data, which can have huge privacy implications.
E.g. let's say we both work at the same company and get the same salary statement PDF aside from the dollar number & your name (which I know).
I can simply brute-force craft a file that changes that number around and upload it to iCloud, when I stop paying for storage I know I've cracked what's on your drive.
In any case, I'd be surprised if Apple's not already leaking this information due to caching in a way that could be revealed via timing attacks.
b) It'll lead to hugely erratic pricing for consumers. E.g. let's say you download 100TB of movies from BitTorrent, now you pay almost nothing for it, but if everyone else deletes their copies pricing for you will go up.
Apple could mitigate that by never raising the price on a given chunk, but that just leaves them paying for it, and it's easily abused. Open two accounts, upload the same data, then delete it from one account, pay 1/2 for storage.