Earlier quoted context omitted.
That's the big question, with a universal application: that tension between the fact that knockoffs have a diluting effect on the original, and the idea that if your idea can so readily be copied it's perhaps not worthy of protection.
The magician's creed is a lot like an open source license with the restriction of attribution. If attribution is given then copying is considered respectable, whereas if it's not then it's considered theft.
The general rule is you are to ask the individual you are copying if you can copy them in which case they will say no unless they sell the trick, or you can argue that you independently came up with the trick, and the person agrees with you.
Copying is always considered "theft" even with attribution unless there's specific permission.
There is a case where a performer does magic to the same song (Shape of my heart), but with different actual magic performance and it is considered copying.
*Edited to improve my tone, the person I responded to deserves good faith and seems quite reasonable.