This seems to be an issue with KF8 ebooks on the Paperwhite. I've ran into the exact same problem with "Times New Roman" being defined using an inline style on virtually every html element. This seems to prevent the user from changing to any other font. Any user agent, whether it be a web browser, ebook reader, etc., shouldn't behave that way; the user should always be able to select their preferred font.
For now, the only solution is to do as you have done by converting to mobi, request a refund and post a scathing review about poor publishing, or try to contact Kindle customer support. Hopefully this will be rectified in a future update.
See:
https://www.mobileread.com/forums/sho...66#post2398666
https://www.mobileread.com/forums/sho...8&postcount=15
Edit:
I don't believe it was the publisher's intention to lock the ebook to a specific font. Properly made ebooks with embedded fonts do not behave this way.