I believe your guess also correctly explains exactly why AFMA is not included on the new Rebel SL1: useless to most users, not a lot of requests for it. So the manufacturer makes the most sensible decision.
We'll just have to agree to disagree on this one. I think it's at least as easy to understand and useful to the Rebel user base as 2-axis white balance!
You're avoiding the question about whether the useful "edit CSS" feature should be offered to all WordPress.com customers,
Thought the question is not terribly relevant here, I did address it though maybe I didn't make myself clear enough. Yes, of course the marginal cost of enabling that feature for the remainder of the user base is either zero or very close to zero.
In software, marginal costs of expanding or upgrading the user base are nearly always close to zero, but you will always lose if you price near or at marginal costs (because marginal costs are zero or close to zero whereas fixed costs are very high).
So the answer to your question which is, essentially, "why don't software vendors price at marginal costs" is:
"because marginal costs are dwarfed by fixed costs. If they price at or near marginal costs they don't recuperate their fixed costs).
Also, in this particular case (wordpress), it is most likely that the non-paying users if not free riders are not the primary source of profits (actually IMHO they are pretty close to free riders). This is not the way it is with cameras, where (a) the marginal costs are not at all negligible, and (b) the cheaper models are a major (perhaps primary) source of revenue that the manufacturer can use to offset fixed costs.
