I think they chose the WEIRD acronym to challenge the western centrism. For most of the readers of the book, the culture described by the WEIRD acronym is not only the normal culture, but is in many ways considered universal.

By calling it WEIRD, the author is trying to drive home the point that the vast majority of people in the world are NOT in that culture that many westerners feel is 'normal', which would make it 'weird' in the sense that it isn't actually the norm.

Now, I have a lot of problems with the book and his arguments, but I don't think there is anything sneaky or nefarious about the word choice, it is very up front and straightforward as to the reasoning behind it.