that's a lot of assumptions you have there.
a) I have a non-ad kindle
b) I want to opt in for ads that I did not even sign a contract for
c) I think it's a fairly subjective topic. It might be dishonest from a contractual point of view but as I would be actually looking at the ads at my pleasure, Amazon still gets it's pay and the companies who benefit from advertising do as well as I do buy their advertisement if I feel like it. I might be even less annoyed at them as I don't have to see them all day on a banner.
In all, the modification does not effect anyone but myself, unlike 3g tethering, or complete ad dismissal.