No, herb grinders are not illegal. This is because herb grinders or simply grinders are used to shred herbs and spices. It is a cylindrical device with a top and bottom with sharp pegs that, when turned on each side, shreds the herbs and spices.
However, it is worthy to note that herb grinders are sometimes (most of the time, in some cases) used to shred marijuana which can be used to make a joint. This is when herb grinders become illegal in most countries and states in the United States – since, of course, marijuana is considered illegal. However, it is also worthy to add that some people who have a license to buy marijuana for recreational purposes also use herb grinders to make a joint.