Just a few hastily composed thoughts here. Setting the default state for conference presentations to "do not share on twitter" is both absurd (given that the whole goal of academic knowledge production is, you know, sharing and discussing knowledge) and impractical (given the rather limited affordances twitter has built in for users to censor one another).
That said, I think it might make sense to take a step back and ask ourselves: what on earth would motivate someone to make the ridiculous and seemingly self-limiting attempt to stop others from discussing their work? Framing this as another confrontation between those who do and do not "get the internet" seems to be a limited frame, in my opinion.
Instead, I can't help but wonder if this desperate attempt to lay claim to knowledge as property is driven by fear. The act of an increasingly precarious academic population desperate to hang on to any asset, however intangible, that might give them a leg up in the dog-eat-dog academic job market. Of course people resent what looks like the "theft" of their reputation. They experience reputation as a scarce asset. One to be guarded jealously.
This attempt to hoard reputation, to hoard the fruits of our intellectual labors, makes us all poorer. In a sense, this could even be seen as analogous to the "paradox of thrift" familiar to Keynesian economists. If everyone fears for their future, and everyone over saves, the economy collapses.
But Keynes did not suggest we simply shame savers into spending. Instead he emphasized the need for collective action to drive investment. I am a strong supporter for openness, and I think it must be first among the values of our profession. If we truly want to create an academic culture of open ideas and shared information, we must take steps to secure the material conditions where people feel secure being open! I think this interpretation of openness is somewhat in contradiction to the sometimes crypto-libertarian language that often infiltrates our discussions of free information. It is not enough to "set ideas free." We must build safety nets (with all the slogging bureaucracy that will entail) if we want people to feel genuinely free to share and to fail.