Circuits can be protected, but usually by patent.
People who try to protect a circuit usually try to protect the design of the circuit and not really the function.
A lot of circuits are actually provided free by the companies that make the components.
Take the PT2399 delay chip. One company that manufactures them is Fairchild Semiconductor. In the datasheet, the detal codument of the chip, they actually give you the schematic of how to build an audio delay. An enormous amount of delay pedals are built around that schematic. Fairchild doesn't care, they just want you to buy the chip, so they give the recepie away to make you use the ingredients.
Other circuits, like distortion, are the same way. A lot of Op Amp makers give the the recepie and EVERYBODY uses that recepie to make their distortion.
If you don't use an Op Amp for distortion and instead use diodes and transistors it's the same thing. Saying that it is illegal to make a Big Muff clone is like saying it is illegal to make ravioli because Olive Garden makes it.
Apparently the only guy who owns a Shamray.