This is because prices can change very rapidly with ebooks depending on how the book is doing at the time.
The publisher sets the price of ebooks on Amazon wheras the seller sets the price of Physical books. So with ebooks some books have free promotions that may last weeks, multiple days, and some that expire in just a few hours.
If you see a book you like and it is currently free - download it now.
All a free book costs you is the space it takes up on your kindle or ebook device and if your an avid reader then it makes sense to download them as you see them.
Sometimes the promotions last less than 24 hours. These are all books that are worth reading, so even if you do wind up purchasing one for more than 0.00, you should be able to get an enjoyable read in.
Why not visit Kindle free books.co.uk and get a new free kindle book every day?