@emilis
1) Irrespective of whether it's right or wrong to arrest people over accepting the King, I think the caller is right that the barista was goating the police officer. Irrespective of what the potential crime is, you don't get to play around like that. You don't get to call in a fake bomb threat and then pretend you were only joking. You don't get to say "I was only pretending to stab someone, officer". I don't care if it's justified for something to be a crime or not, you do not get to play around like that, you do not get to waste the time of the publicly funded police. I don't care if you were only pretending to litter, you absolutely get arrested, to teach you a fucking lesson, that law enforcement isn't a joke to entertain yourself with.

2) The unrest is attempting to call for a revolution and complete reform of your political system. The host seems to be either ignorant or disingenuous about just how unstable things could be when the Monarchy changes hands, in a world where monarchy isn't the norm anymore. The King isn't elected. He doesn't have the public confidence bestowed unto a politician via election. He needs to prove himself before the people as a king, and he can only really do it after being given the title. If there's a time revolutionaries can strike, it is right now. There is no better time for dissolving the monarchy and "eating the rich" than right before a new king has time to prove his worth.

3) Holding a sign saying "long live the king" at a republican gathering is akin to holding up a sign saying "god is real" at an atheist gathering. I fully expect the atheists to not be offended simply because we live in a society where atheists are aware and understand that the concept of gods has existed for countless generations and that there are still people who believe that. Even if they vehemently want to abolish the old religious order, they still understand that this is the current norm.
But holding up a "there is no god" sign at a Christian gathering is a whole different can of worms, because you're intentionally trying to upend the status quo. For a Christian, that's gonna be a foreign concept and message to see. It not only goes against what the current societal norm is, it goes against historical precedent that people believed in gods.
You could rightly say that the monarchy supporters shouldn't be protected from offense. But I disagree that the situation would be equal if a similar act of offense was done towards republicans.

4) I don't know what the media spin around all of this is, but I don't think the real underlying reason for these arrests is offence. The underlying reason I think is: "our state cannot afford a political upheaval right now, we cannot afford a conversion to pure republicanism right now, we simply can't let anti-monarchy sentiment spread in this moment in time".
I'm sure there have been countless anti-Queen sentiments and messages in the past that people have ignored. But when the context changes drastically, suddenly something that was once harmless can be very harmful.
So it absolutely is "very exceptional circumstances". That's how the world works, unfortunately. In exceptional circumstances you might need to take exceptional measures to assure a safe transition back to normal circumstances.

5) I can perfectly agree that these people shouldn't face arrest or a spot on their record for this, but if you're gonna pretend that a "this is not my King" message today is the same as a "this is not my Queen" message a few weeks ago, you're absolutely bonkers.