Intellectual rights to magic methods
Several forms of Intellectual property protect magic methods.
Copyright
Magic secrets (referred to as methods by magicians) are not directly subject to copyright, since they are abstract ideas rather than realised expressions. The relevant section of the Copyright Act reads:
In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
This is not a "loophole", but a deliberate act intended to force any such methods or similar to pass through the (more rigorous) patent process before being granted protection. However, the description of a magic method is a realised expression, and thus may be copyrighted; and the actual magic act performed by a particular magician may also be considered a realisation.
This means that, in most cases, revealing a magic secret - even one that is on sale elsewhere - is not a copyright infringement, provided the description is not a verbatim copy of another description that is otherwise available, and does not include details of a particular magician's stage adaptations of the trick. It is worth noting that many of the books, DVDs and videos on sale to explain magic tricks are not sold by the original creators of those tricks, thus no money is being passed back to the inventor; so objections related to "depriving the creator of money for their hard work" do not apply.
Patent
Magic tricks can indeed be patented and some (such as Pepper's ghost) are: however, this is resisted by many magicians as the trick's method would then have to be disclosed and held on public file as part of the patent application. Even if a magic trick is patented, the patent does not prevent anyone from revealing how the trick works, the patent only prevents practicing, performing, or making a device to perform the trick without permission.
Trade Secret
There are also laws related to trade secret and contract which may apply if a person exposing a magical method has a) been required to sign a contract when they learned the trick, or b) been taught the trick by another magician, who did so on the understanding that the learner was a magician planning to use it in their act.
Although trade secret varies from state to state and country to country, it only protects information that is not publicly known and is intentionally kept confidential. For example, any method that is available in the public library is obviously not protected, while a method that is known to a small number of magicians is.
Ethics
It should be noted that there are ethical issues associated with exposing magical methods publicly. The International Brotherhood of Magicians' Code of Ethics states "All members of the International Brotherhood of Magicians agree to oppose the willful exposure to the public of any principles of the Art of Magic, or the methods employed in any magic effect or illusion."
Any individual who is, or is hoping to become, a professional or amateur paid magician is cautioned that performing exposures may result in exclusion from, or damage to their relations in, the community of magicians.