As the term suggests it's kind of like sex. People have sex all the time and usually do it for certain generally accepted reasons like expressing affection, having fun, and procreation. Sometimes people do it for money but that is so taboo in many communities that thousands of people are punished anually for explicitly and specifically exchanging sex and money. Even though it goes on implicitly all the time.

Now here's a website focused on perl, with a voting system that didn't always exist. People post here in order to learn, teach and also to provide their peers with a certain kind of browsing experience, focused on perl and enlightening (or something, something good). Xp and rep are just reflections of the information generating perl centered activity, like money, and it's somewhat taboo to exchange the activity explicitly and specifically for xp. Even though it's normal to receive xp for the activity!

So to answer then question Why is Xp-Whoring Bad?

It shifts the focus of the site by shifting the reason for participating.
It puts the cart before the horse.