However, when it comes down to it, even if the agents started the plot, these guys went along with it. They did not leave, they did not report to the police that the group was planning something illegal.
That's pretty much what 'entrapment' is about.
You, hopefully, don't want the authorities to be creating criminals just so you can capture the criminals that wouldn't have existed without you creating them. Talk about an abuse of power and a counterproductive waste of taxpayers' money!
Which means you shouldn't be wanting the agents to be "starting" a plot. You shouldn't want agents to be *creating* criminals, just so they can capture criminals. You don't want your firemen to be arsonists, you don't want your pest-exterminators to be pest-breeders, and you don't want your cops to be entrappers.
Encouraging someone into a crime, even when the benefits you plan to extract from it is arresting them for the crime (rather than the fruits of said crime itself), should be itself a crime equivalent to the crime encouraged.