how many of them have that goal? what do you think of the monitoring of the rest of them?
Of course, that's true of every organization. How do you know that the Kensington Garden Club isn't plotting the violent overthrow of the British government? But there's some level of plausibility where you should investigate. (Note well: I am not claiming that the British police drew the line in the right place.)
I would add that enterism is a real thing with quite a few notable cases of a minor fringe group within a moderate party serving as a spearhead to takeover the reigns and leverage its goodwill to advance extremist ideals. This sort of thing is only caught by insiders keeping tabs on the organization's inner workings.