I think the different countries mentioned as being supported may have more to do with licensing, than anything else. Even if a book is temporarily free, it doesn't mean the publisher wants it distributed globally, for free, and may exert some control over it, allowing it in some countries, but not others.
As far as taxes are concerned, it may all depend on why an item's final cost is $0, as to whether there are any taxes due on it.
I can only speak from the perspective of where I live, since I am not a global tax expert.
In my home state of New Jersey, there could be taxes due on a $0 purchase, depending on how it ended up that way. If, for example, Amazon marked down the price for clearance or just a sale, then it enters my shopping cart already priced at $0, and no taxes would be due on it, because 7% of $0 is $0.
But if it entered my cart at $2.99 and the use of a coupon or gift certificate reduced that price to $0, there would be taxes due on the original price at which it entered my cart, because 7% of $2.99 is $0.21, which is more than $0.
So, the online merchant, if they are required to collect the tax, would have to do so and then turn it over to my state's government.