Not sure about in the US, but i think in the UK the rule is that they have to give you back the tax if you ask for it, but they do not make it easy for you to do. i.e you might have to specifically request this in writing.

some low cost carriers though (e.g. ryanair) have gone even further and have put an admin charge on refund of taxes. Since the admin charge is higher than the taxes you would get back, there is no point anymore.