/s neuters sarcasm completely. In my opinion if you won't commit to being facetious online because you're afraid someone will (inevitably) think you're being serious, then don't do it at all.
(I can't quote the specific rule, but I seem to remember this from some feedback that @dang gave me a while ago.)
"Avoid snark" is my first guess.
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."