It seems Simon & Schuster has different relationships with the various country platforms for the major ebook sellers. It seems that in some countries the promotion is on the audiobooks rather than the ebooks.
It’s usually not actually US only, just the exact $US 0.99 promotional price. (It would be great to have specific listings by country.)
In Canada, Amazon.ca typically has the same deals for $CDN 0.99 and I’ve seen the same but £ 0.99 at times on the UK site.
Checking the current Amazon Kindle prices for Canada shows the ebook deal $0.99
The UK Amazon Kindle is showing some of the books at £ 4.99, others are at 99p - but has Audiobooks for the Genesis wave showing as £0.00, that is for free.
The German Amazon Kindle de site has the ebooks also seemingly at full €8.99 for some, but others are going for free with a €0.00 listed price - including the Voyager ones - but again the Genesis Wave promotion for free audiobooks is in effect.