I realize that not everyone is interested in the philospohical implications of language, so I will be brief and then drop this (although I'll be happy to discuss it in a forked thread).
(1) Redefinition of existing terms is part of the ongoing process of language development. In Shakespeare's time, "prevent" meant "to go ahead of", for example. Within our own lifetimes, "owned" has grown a very different definition from "possessed".
(2) Definitions of terms themselves are not, and cannot be, objective. There are places where a Dr. Pepper, for example, is called a Coke. One can question exactly where the sun ends...is solar radiation part of the sun? Depends upon how you define the term. There are philosophical questions that examine this far more deeply, but suffice it to say that "objective definition" is an illusion, even within a given cultural group.
I used to own a DVD player that was a television.
Likewise, some folks would say that 3e or 4e is not D&D, because D&D already is something. When WotC calls 3e or 4e "D&D", they are redefining the term, and some folks think that redefinition is objectionable. I am not one of them, but I have no difficulty understanding their position.
RC