I find it most disturbing that they are polluting the global namespace with all those variables and functions. Don't JavaScript developers these days understand how to properly scope their JavaScript?
Reusable components and utility libraries is something different, of course.
I'm just guessing, but the site was probably not developed by a technical team, probably some web shop that specializes more on the design/layout/content side and not on the JS side.
So it's likely it's the work of a non-developer than a bad developer. The net result is the same (bad code) but the distinction is that the code was possibly written by someone who wouldn't even be considered a developer.
a) Because it isn't "right" as such. It's just "better" if the code is going to be reused. Which it isn't.
b) The code style serves as documentation. I can clearly read from this code that it's not a reusable component. It is now easier for me to get an idea of how and where this code is being used in the site.
c) Personally, I'm wary of "pre-emptive generalization". Every layer of abstraction makes the code slightly less readable. Also, unless there exists at least two use cases right now, you're not likely to get the reusability right anyway.
To understand how these problems can be addressed, it is instructive to consider an analogy. Imagine, for a moment, that somewhere in the basement of the Pentagon there are two computers. They are identical hardware, and they run identical software; they are as alike as two peas in a pod. On each of them, there is an executable procedure called "DestroyBaghdad". On one of the machines, executing this procedure does nothing. On the other, the effects of executing the procedure are disastrous, at least for the citizens of Iraq. The only difference between the two machines is a single wire coming out of the back of the second machine.[1]
The point, of course, is that nothing inherent to a computational language is dangerous; danger is posed only insofar as the effects of executing statements in a language include "side-effects" in the external world. If all the "wires" connecting a computational language to the outside world are cut, then executing programs in that language is harmless. (An exception to this claim is that such a program can still tie up the CPU, virtual memory, or swap space on a machine by going into an infinite loop; this can be handled via resource limitations.)
[1]It should be noted that no ethically-trained software engineer would ever consent to write a "DestroyBaghdad" procedure. Basic professional ethics would instead require him to write a "DestroyCity" procedure, to which "Baghdad" could be given as a parameter.