If a third party can build a box to decrypt your encryption without you providing a key, then instead of suing the third party out of existence, you should pick a new encryption algorithm that's actually secure. Consider this analogy: should Master Lock be able to sue companies that make padlock shims, since several of their locks are vulnerable to shimming attacks?
The companies that made the lockpicks have the argument that they're only to be used by professional locksmiths in the process of opening a lock that the owner has authorized them to, which is legal. Selling a tool that only has illegal uses would quickly lead to trouble.
The third party clients can only be used in a way that violates the ToS.
Building and owning circumvention devices is generally legal; using those devices to access things protected by locks, which you are not authorized access, is generally not legal.
