I think they're trying to be seen as against societal rules, out of bounds - that they're not dumb, they're just being rebellious/quirky - so they make it obvious that they aren't using punctuation on purpose. In reality it just makes them look stupid. I see a lot of communities engage in this though, not just the fat ones. People typing like toddlers is an actual thing in some groups....lik diz iz wat u'd c in a rlly shitty txt chat