It all comes down to the end user agreement you agree to when purchasing the ebook. Content providers are within their right to define how they want their copyrighted works to be used - though a judge could potentially throw out the end user agreement in court.
So, if Amazon stipulates that you should not tamper with or remove the DRM from the ebook, it's a violation of your purchase agreement if you do.
Basically, you agree to not remove the DRM when you pay them for their content. The only legal alternative is to not buy from them.
I'm not a lawyer (so correct me if I'm wrong) but here in Poland there is a law that allows you to reverse engineer a product that you bought to make it work on your hardware (e.g. you bought a windows only program but want to make it work under Linux)
So I I buy a book that can't be viewed on device of my choosing (e.g. because of DRM) then I'm free to do whatever I want to the product to make it work on my hardware.
No EULA can be above the state law.
In the UK at least, there are a variety of reasons why a contract, or at least part of it, may be unenforceable - such as unfair/unreasonable terms, things that contradict the law, or the particular terms were considered "hidden" (e.g. too small a font size, or an important clause is buried in page 27 of an unrelated section).
So, if Amazon stipulates that you should not tamper with or remove the DRM from the ebook, it's a violation of your purchase agreement if you do.
Basically, you agree to not remove the DRM when you pay them for their content. The only legal alternative is to not buy from them.