As long as it gets more difficult out in the boonies... It shouldn't be impossible, of course, even out at the recently inhabited areas, to bug a choir, but the images they get should be prone to being misinterpreted by any involved--including the astropaths themselves. Still, the images are types that are made for the human mind to interpret, so there is that...
And if they bug an astropath as they report the message to others, they will get the difference between what they say they see when interacting with the choir, and what they report...
Also, not all of the bugged choirs are near the center of the Imperium, right?? Some of the stuff the Culture is doing is towards the edge?