If a team is using "secret" signals, out in the open...
I don't think it's cheating to use some effort to figure out what the signs mean. If I start yelling instructions to my team in a foreign language, I've got to expect someone may know, or try and learn, that language. My use is out in the open and I'm gambling someone can figure it out.
But if I'm taking pains to hide the signals I'm giving (like a catcher signaling the pitcher by making a sign between his legs), installing a camera in the outfield to see what the catcher is secretly signaling does, in fact, feel like cheating. Same as installing a microphone or camera in the visitor's dugout.
And if my league says it's illegal for a runner on 2nd to relay signs back, well, then it's cheating.