the goal is communication is not always clarity

the fun of sarcasm is knowing enough to recognize it

if somebody is too out of the loop to recognize it and picks an argument, it's collateral damage

That may be so, but in normal communications, such as speech it is tone of voice and possibly non-verbal hints that will indicate sarcasm. Online you don't have that and the 'collateral damage' can result in a lot of wasted bits from people reacting to the words rather than to the joke and this in turn can drown out the rest of the discussion.

Adding a /s to a post is the internet equivalent of saying "just joking haha" after saying a joke. It ruins half of the fun.

I understand why you might want to add a sarcasm indicator to texts including sarcasm. I'm not arguing against that point. I'm trying to show the other side of the coin: why some other people would not want to add those sarcasm indicators.