Several publishers now set their own price for Kindle books, not Amazon. Which often means pricier than what Amazon was charging. And now Amazon says they're adding sales tax on any book where the publisher sets the price.
Here's the full damage, from a message posted in a Kindle community forum by Amazon Kindle Customer Service:
Several publishers have recently changed the nature of their relationship with Amazon, moving to a business model whereby the publisher, not Amazon, is the seller of record for their books. Kindle books sold under this model are subject to sales tax based on the publisher's state tax reporting obligations and the taxability of digital books in those states. Books where the publisher is the seller of record say "This price was set by the publisher." Nothing has changed with respect to sales taxes on Kindle books where Amazon is the seller of record.
Basically, if a publisher is setting the price, Amazon is collecting sales tax based on the state where the publisher resides. The publishers at issue here, as TechFlash discovered, are Hachette, Harper Collins, Simon & Schuster, and MacMillan, and it varies from state to state, so check out the full list.