I will agree that TKD gets much more than its share of McDojos. There are at least 10 TKD schools in my city. I was on a break from Kenpo (instructor was too far away for me to commute), and was just looking for something else to learn, since I couldn't find a kenpo school in my area. I visited all the martial arts schools listed in the phone book. EVERY SINGLE ONE of the TKD schools was a McDojo. I walked in and the first thing they said to me was "let me tell you about our two year black belt program. its guaranteed" My answer was "black belts should be earned, not bought" and walked out.
There are good TKD schools out there. I used to train at one in Los Angeles for a little while, so I know they exist. I just wanted to learn a couple of those fancy kicks, not because I'd use them in a fight, but just because they looked cool and I wanted to know them. The instructor and I had a long talk. He lamented the opening of the McDojo down the street, because he was losing a lot of potential students who didn't know any better. They would come into his place and see that he didn't guarantee black belts, and would go down the street and see that the McDojo did guarantee a black belt. He shook his head and said "I can't make them see that you can buy a rank, but you can't buy the knowledge that should go with it."