Exactly, that's the issue there. We've got people in this thread seemingly unaware that protestors could block roads or spray paint... as if no protestors have ever done that before. Apparently, a police presence magically solves that... but funnily enough it doesn't necessarily when don't stop oil types try it likewise football hooligans don't magically all stop being violent just because police are present at matches and sometimes they need to conduct surveillance on the crowd, ban certain people from matches, make them stay at home during European matches etc.
Likewise arresting people for going equipped is necessary, if you find some don't stop oil protestors with a load of glue and/or paint then it's pretty obvious what's about to occur and it's better you're able to arrest them before they cause damage/disruption. Ditto to a known burglar found with a set of lock picks.
None of this is particularly hard to follow but some posters are seemingly confused by it and/or pretending that people were arrested for simply protesting. The other issue of course is that the police were overly cautious here... which is quite easily understandable given the super obvious context of the event. I don't think they'd be as cautious re: people turning up at the regular changing of the guard ceremony for example. Likewise the security precautions in place for say a Joe Biden visit will obviously be erring more on the side of caution than say a visit by a local mayor to open a new leisure centre. Of course, that context is ignored too.
The lone women/royal fan who was arrested was clearly a screw-up, the protestors on the other hand much less so, they'd have far more flack if they messed up and some loons blocked a road or sprayed paint over the coach or managed to cause an issue with the horses pulling the coach or indeed the cavalry guys via setting off rape alarms or whatever the plot was supposed to be.