> Why on earth would you want to prosecute the supermarket where ICE buys loo roll? The builders merchant who sold them fence posts?

In mafiya/organized crime, (serial) murder cases it's common to prosecute knowing supporters as "aiding in criminal activities". The supermarket cashier selling loo rolls to the ICE may not be a "knowing supporter", but the fence post seller certainly is.

As for the people involved: there won't be justice, not as long as the US has nukes and refuses any international control.