> I can't quote the specific rule,

"Avoid snark" is my first guess.

I don't really think that's comparable. Tone has more nuance and is a different medium of communication than the literal words you're saying. There are two channels of delivery there: the tone used to signify sarcasm, and the words you're saying. Tone is more subtle and you can modulate it to be more or less obvious. For example, you could opt for a dry delivery using little to no sarcastic tone (which is what typing sarcasm without /s is closest to, in my opinion).

When you use /s, you're disclaiming the sarcasm immediately and in the same delivery as the words. It doesn't resemble tone at all, because you can't choose to be subtle. You don't get a beat in which the other person is initially surprised and then "gets it."

:) welcome to the great divide between spoken British-English & spoken American-English