Realistically, three things can happen to people with App under this policy. Take, e.g., Kindle.
1) They withdraw the Kindle application 2) The add In-App purchasing 3) The add In-App purchasing, but with a 30% price hike
Frankly, 2 is very unlikely - 30% is probably their entire profit margin on the sale. 3 is possible, but makes every Android tablet look vastly better to new purchasers ("The same apps from iOS, but with content 30% cheaper!). 1 is also quite possible - they just EOL it and say "For future Amazon purchases, please grab a Kindle or Android tablet), thus showing a huge chunk of Apples target market a big banner for the competition.
I suppose there's another alternative: that this becomes one of the annoying "Not for big player" rules that they just have a sub-rosa agreement with B & N/Amazon to never enforce. That in itself is pretty underhand.
Overall, my favoured interpretation is this is a re-run of the "Nothing but Objective C Apps in the Store" debacle. Masses of bad press, a dent in developer confidence, and a rescinded policy within a month or two. Apple really are their own worst enemy with this stuff.