First, I don't know what they are...and wikipedia does not have an entry related to this...but they often seem mentioned with braindumps.
Second, what are they and why are they frowned upon here?
The study material is extremely close to the real questions and labs... If you don't like legally cheating you shouldn't do bootcamps.
A lot of the bootcamps have a reputation as being designed to prepare you to pass the examination as opposed to really learning the material. Many of them are over a three day to 1 week time frame and they often will host the exam for you on the last today. It is essentially an instructor-led cram session with some other people.
One nice thing I have heard about some of them is instrctors who are willing to stay late and assist with any questions you may have. I know people that have been to them and found them to be helpful. Personally I do not think a week is long enough to truly understand and apply what you are learning, but I can't speak from experience since I have never attended one.
I am sure like anything there are some good ones, bad ones, and average ones. I think a lot of the negativity surrounding them could be a lot of the bad/average ones not being worth the money invested. Another thing could be people with no experience in the subject matter expecting to attend a boot camp and have no trouble understanding all of the material as well as passing the exam (which is how many are advertised). Perhaps some of them also use question/answer training based on actual test questions and actually are cheating similar to the people downloading the information, but I couldn't name any names personally off-hand.
I could see legitimate ones as being very helpful for people with a thorough understanding of the subject matter that needs a strong review before taking the exam. Personally, it is not something that I have ever considered attending.