"Sympathisers" usually far outnumber those who either take any action or communicate with those who do. There are a few hundred million AQ 'sympathisers' around the world. If there are 5000 that have actually taken some action that would subject them either to a FISA warrant or the standard of 'reasonableness' you have expressed, I'd be amazed. I'd also think we need to be making lots of arrests.
We've made some arrests. But I'm guessing out of those 5000, a good percentage are deemed to be non threats and dropped. And of the remaining percentage, it's more valuable to keep tabs on them unless they are planning something imminent. Catch the big fish that way.
Of course with the publicity due to the stinking leakers, it probably scuttled a bunch of leads.