The simplistic logic that the the protesters were infiltrated by the government is irrelevant. In order to arrest anyone before they actually do something is not like being in Russia. You are in America. Civil disobedience is not a crime and even the thought that you can be arrested just for thinking about a crime will not happen. Civil Disobedience is not a crime in itself. It just want happens during the civil disobedience that can generate a prosecutable crime.
So it is not illegal to think about doing a crime. It just makes you a MAGA nut. You pretty much have to catch them in the act or doing something else that is illegal like making a bomb or entering a federal building caring a concealed weapon. This gives the legal system a motive for the crime and the means to commit a crime. Guilty as charged.
Talking about it will probably not work in the US legal system in obtaining a conviction. Even with testimony from informants.
So protestors are doing civil disobedience. No body plans for a riot well who knows what some of these guys were thinking that they could do in broad daylight. It just happens when things get out of hand or some are a little imbalanced in the head. If they were making pipe bombs and carrying weapons, then yes law officials can arrest them before doing the actual crime. They will be charged with various infractions of the law. If if is foiled , the prosecution can point to what might have happen if they weren't stopped based on their actions up to the point of arrest.
So if the rioters were just going to march up and down the street then that is it. Informants can not have them arrested. IF they do more like rioting , breaking and entering. assaulting police officers and the general public, carrying a concealed weapon or bomb, then they will be arrested because now they did a crime.
Not one was arrested for planning to do something, They were arrested because they did something that was against the law. The planning just gives the prosecution a motive that they can pursue. There has to be other things such as damage, assaults, threating violence, etc.