I'm not sure where I stand on this issue of conversation privacy in public spaces, but I will say that at worst the individual should have called the people making jokes out at the time and suggested that their conversation was not appropriate in context. To passively aggressively take to twitter to gather a mob strikes me as total overkill.
The distinction is the same as if they were seated in a coffee shop. Conversations there are between private people, not with some coffee shop "audience". People who listen in to such conversation, and worse, reacts to it, are doing something which polite people consider very rude.