Are any publishers really offering those kinds of terms, even to Amazon?
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.
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.
So: you (the publisher) guarantee not to sell ebooks for less than $9.99 elsewhere, and not to publish a dead tree edition for less than $12 ... and Amazon are free to cut their competition off at the knees, sell books for $6, drive their competitors into extinction, and then can raise their price to whatever the market will pay.
Meanwhile, as the author's royalties are a percentage of the SRP, that 10% of SRP cut they're due -- $1.20 of a $12 title -- comes out of whatever Amazon pays you -- $4 in the case of a book they choose to sell for $6 on Kindle. The SRP is set to reflect the price at which the publisher expects to sell enough books to gain sufficient sales to break even -- assuming a 40-50% discount. If the discount goes to 70%, they're in the stinky stuff up to their eyeballs. That $12 ceiling is forced on what was formerly a $24 SRP, and a $14-16 expected sale price. The money has to come from somewhere, and it ain't going to come out of Jeff Bezos' wallet ...
What Amazon have done is to sneak up on the distributor/bookseller pipes and merge them into one lucrative hose, and now they're playing both ends for their own benefit.
Amazon squeeze their suppliers, just like Wal-Mart. Amazon is already corrosive -- if you're a small supplier.