Is this claim backed up by any actual evidence? Does Amazon pressure self-published authors the same way they pressure publishers?
The claim is not that Amazon is pressuring self-publishing authors today - it's that they could start in the future, as an ebook monopoly would give them have the means, motive and opportunity.
The only authors that have to fear that future are those that produce works uninteresting enough that people won't look for them outside of the Amazon store.
Does this mean Stross is an uninteresting writer? Or is he worrying about nothing?
Maybe he is. SF has never been particularly interesting to the general reading public, alas.
If your list price is above $9.99, you get 35% royalties instead of 70%.