So who is responsible? I can’t claim familiarity with Dutch law (although I’m assuming tulips have special rights), but I’ve argued before that in the United States the First Amendment protection of free speech extends to robots—including bots. It would be unconstitutional for American police to require van der Goot to shut down the bot if its speech qualified as constitutionally protected speech.
“True threats” are not protected under the First Amendment, and the Amsterdam police certainly considered the bot’s tweet just that. But it’s hard to say whether the tweet expressed sufficient “intent to intimidate” to qualify as a true threat without having more information about the tweet, the bot, and the circumstances around it. In Virginia v. Black, the U.S. Supreme Court ruled that the government “may choose to prohibit only those forms of intimidation that are most likely to inspire fear of bodily harm.” If the bot’s tweet was not reasonably likely to inspire fear of bodily harm given all of the facts, the Constitution would have prohibited police efforts to delete it, even though it’s not a person. If the bot’s tweet falls within constitutionally protected speech, no one would have had criminal responsibility because there would be no crime.