Not going to name any names. But i have heard quit a few well known magicians claiming copyright and performance right for their effects.
If someone has made a statement to this effect, then point it out. They have gone on record. Now if someone told you this privately then, one, it is hardly of mythic status and two that person is just an idiot. To not back up your claim that this belief is pervasive only makes it look like you are making stuff up that you don't know anything about. As I said, I have been in this game a while and have never heard an informed magician make any claim about there being a copyright for a trick.
Having said that, the british theatrical unions had some interesting policies in place that restricted who could perform certain tricks on their stages. McComb had a great story about this. While it wasn't a copyright, it was a form of protection.
Likewise, Tom Stone has written a bit about Swedish ip laws which are different than those in the US and stronger when it comes to a creators rights. For example, according to Tom, if a magician performs an effect in a mixed media (video tape, for example) then that effect can be considered (like some forms of choreography) protected. There are some interesting threads with contributions from real lawyers on these matters over on the genii forum.
Now, performance rights are a different issue and have to do with routines. If you know anything about theatrical liscenses then you know that buying the script for a play, like Wicked, does not give you the right to perform it.
Likewise, just because you own the sheet music to poker face doesn't mean you can legally record it or even perform it in public.
So, magicians CAN reserve and/or grant performance rights to their material as long as they go about the process properly.
So, while there is no copyright on an idea, there are copyrights on specific incarnations of that idea. There are also some other legal protections available, and creators can restrict and withhold certain rights as theatrical agencies do (though this is rare and seldom handled properly).
But I think saying there is exists a widely held belief that a trick can be copyrighted is an overstatement.
At least - among people that know anything either about magic or ip law.