There is no law in any country that covers the whole range of what we call currencies, but there are many laws that cover aspects of them. For example, in the U.S. there are federal tax reporting regulations about barter exchanges. Some uses of free currencies may be seen as breaking these regulations if participants don’t do the necessary reporting. But notice that other countries have no such laws, so there will be very interesting legal cases around transactions in open currencies between people across national borders. Another example: certain countries and many U.S. states have laws that cover businesses issuing coupons. Some free currencies may be seen as coupons under these laws and thus be subject to them. So the legality of a particular currency will depend on its particular rules of operation, and how the governing jurisdiction interprets them.
The fundamental challenge is that free currencies are a paradigm shift. They require a new understanding of how currencies are actually a large family of related wealth building information systems. They require agreement that creating these systems is a fundamental right of all communities. This paradigm shift creates new social evolution, laws will follow much later. This is the case for the abolition of slavery, racial and gender equality, freedom of speech, freedom of religion, freedom of assembly, etc.
So, on the one hand Nations claim their sovereign right to issue money in the name of citizens, but on the other hand this right has been abandoned to the private banking sector. On the one hand Constitutions declare the rights of citizens to equal treatment and safety, but on the other, conventional money works in opposition to these declarations. Open currencies will hopefully, in time, release us from these fundamental contradictions.