I think the reasoning behind a question like this is as flawed as believing there is something *off* about straightening relaxed hair. As if there's only two styles in the world and they can only be achieved in two ways. Relaxed for straight and natural for kinky/curly/coily. There's a lot of grey in between. I think one of the greatest things about natural hair is that you CAN do so many styles.
I know heat is damaging but it is just a type of manipulation just like using certain products and techniques to create different looks other than your hair in it's natural wash and go state (gel, wax, shine products, holding spray, braid outs, twists, etc). They all create a look that changes what your hair looks like coming naturally out of your head. Some more involved than others but to some degree, the same. What's wrong with that?
As a previous poster said, it's the same as other "natural" non-blacks who have relatively straight hair getting their hair blown out/straightened. It's a different look or may be viewed as an enhancement and I can understand why some find it desirable/more desirable and easier for THEM to maintain.
Having natural hair is not (necessarily) about making a statement or showing off that you're natural just because. Maybe to some it is but to some people, it is what it is: the stuff that grows out their head! And just like anything else on their body, they can do with it what they want. It's an accessory, an added beauty and just like other forms of beauty - like clothing or jewelry - it can change, have different styles, etc.