It is a short answer and gets us out of writing a big, long-winded piece on the legalities but perhaps inadequate.
We are not alone in our opinion. We direct you to a great article by great Magician and Lawyer Guy Hollingworth from 2008 about an art designed to avoid laws of nature.
It is comforting to see that the country from which we have derived the greater part of our legal system, has not backslid into the easy but philosophically unsound world where an idea can be protected. The United Kingdom wants to encourage innovation but draws understands it must draw the line somewhere.
In the case of magic tricks, one can patent the method to perform the effect or even copyright the patter used to describe and deceive; but one may not protect the idea behind the trick itself.
For instance, the secret behind our now-classic Marked One-Way Forcing Deck can be stolen by just about anyone. Of course, some print critics of our invention have suggested “[w]hy would anyone want to steal the idea of a One-Way Forcing Deck that is marked as well?” Regardless, it is not being knocked-off or copied by folks looking to cash in on our genius. We like to think that is because our brothers and sisters in Magic are ethical folk.
By the way, we will soon announce the follow-up to the Marked One-Way Forcing Deck, The Inside Magic Marked Billiard Balls. No longer will you have to guess about the location of any particular billiard ball whilst you make them appear or disappear.