This is clearly incredibly complicated and hinges on all kinds of nuanced definitions that are not yet universally accepted, such as "what is twitter?" As a thought experiment, though, if Twitter is not a publisher, then I think it would be acceptable for the government to sanction it for failing to provide "equal time." That is not the same as the government sanctioning it for airing or failing to air the type of content that the government wants. The latter is clearly an overstep, but the former is currently accepted doctrine.