It is widely known that probably THE cardinal rule of magicians is to NEVER reveal the secret to a trick. I especially know this because i'm a bit of a magician myself! In aLOT of the magic books I've come across they usually have an intro with the "rules of magic" or something like that.

Whenever someone comes out and tries to reveal secrets of magic, especially on national television, I would say that many professional magicians dislike it very much! Some professional magicians speak out against those who seek to reveal the secrets. I recommend reading what bill palmer has written about revealing magic:

http://www.billpalmer.com/exposure.htm&e=14702


"Magic is like a butterfly. You can give a 5-year old boy a butterfly, and he can look at it, admire it and marvel at its beauty, and then he can let it go.

Or he can pull the wings and the legs off of it, to see how it works...and all he will have left is a dead butterfly..and he still won't know how it works."

This is so true!

The tricks that Penn and Teller reveal in shows or TV are that of what they have created themselves to be shown to audience members and viewers. Other tricks which they have revealed in the past are of older ways on how to create the illusion of the trick which most magicians dont use anymore. So really in fact they are not revealing anything new or giving away secrets to the way these tricks work.
But... for example the cup's and balls routine that P+T reveal is done in a way that confuses most people anyway, and again this is an old routine which has changed hundreds of times throughout the years.
Hope this helps.

I wholly agree with the previous answer. I would only point out that I have known Penn & Teller to reveal someone's tricks ONLY when those tricks are used to scam people. I've never heard them reveal any legitimate magic.