I can't speak for others, but I will give you mytake on it. I run a Kenpo school. Having said that, I also hold black belts in TKD. Now, I do borrow form my TKD experience. In the area I live in, there are 4 dojos/dojangs including me. There is Gary Crim's Red Dragon (TKD) Zingg's (Primarily Okinawan) Me (Kenpo) and the McDojo in the shopping center(TKD). The reason that particular school is classified as a "McDojo": 1.) High fees for classes and testing 2.) High Belts for low results 3.) No Honoring of people's experience, even within the same art. 4.) The franchise has removed instructors and changed management three times. 5.) They force you into an illegal contract. 6.) They claim theirs is the only "correct way" and do not honor other instructors or styles. That is what I know to be fact about the school. My opinion of what I have seen of the individual that currently runs the school: 1.) His hands are tied by the franchise 2.) He is a highly competent and talented practioner, yet a not so good instructor.
Now, my students have questioned me from time to time about why I have a tendency to bad mouth TKD, even though I am a BB in the style. My answer is simple, the way TKD teaches you to kick, in time, harms your knees and your hips. I hope you have noticed that I in know way bad mouthed Mister Crim. He is also teaches TKD and shares the same views I do in a LOT of areas. We are competitiors, yet friends. If someone comes to his dojo looking for more of what I teach or lives closer to me, he refers them to me and vice versa.
So you see, it is not TKD that I hold in low regard, it is the McDojo in general. TKD does seem to have more McDojos than any other art for a lot of the reasons mentioned, ie. there are more of them than the rest.
Just my $.08 worth ($.02 after taxes, currency exchange and taxes)