That hardcover which I paid $16 for likely has a list price of between $25 and $30. If the publisher was only paid 30% of that $16 sale ($4.80), that means they sold it to the distributor at a wholesale discount of over 80% off list.
Are any publishers really offering those kinds of terms, even to Amazon?
Hardcover list prices are pretty much pegged at $24 in the USA, ever since word went out within Borders (or was it B&N?) about eight years ago to stop buying hardcovers with SRP over $24.
Yes, Tesco (in the UK) and WalMart can and do demand discounts up to 70%. I have heard hearsay reports (I can't cite sources, due to confidentiality) of Amazon demanding 80% discounts off ebooks from British publishers -- which is why they only launched Kindle in the UK about three months ago: nobody would take them up on it.
But even if some resellers are getting titles for 70% off list price, that doesn't translate to them retaining 70% of the revenue from the sale unless they're selling the title at list price. No reseller with the market muscle to demand a 70% wholesale discount is selling those titles at list price!
Personally I think your figures ($24 list price, 70% wholesale discount) are edge cases, and don't represent a typical sale. But even if we take them at face value, in your example the publisher/author get $7.20 of my $16 and the distributor/reseller get $8.80. That's a 55/45 revenue split, not a 70/30 split.
This also assumes I'm buying at 1/3 off list, which is on the low side. Amazon is discounting bestsellers by at least 45%, with a select few going for 60% off or more. Example: Going Rogue, list price $28.99, sale price $13.50 (53% off). If Amazon's getting a 70% wholesale discount, HarperCollins gets $8.70 and Amazon keeps $4.80. That's a revenue split of 65/45 in the publisher's favor.
So: when I buy a hardcover that's been discounted down to $16, the actual reality is that the distributor/reseller is not making twice as much on the sale as the publisher/author -- it's more like a 50/50 split.
None of which is to say I support Amazon OR Macmillan's position in this particular battle (they're both wildly overreaching, IMO). I just think your example inappropriately conflates two different things.
Are any publishers really offering those kinds of terms, even to Amazon?