I think in light of this, I'm going to stick to my theory that the battlecasters are actually golems, or constructs, or robots, or whatever. King DJ just slapped together some code that said "monitor all scry channels", and they don't have the complexity to overstep that boundary. Celesto's right there, but the domain they're looking for him in doesn't extend to "right here".
It's also why you can even do something like "watch all seers" without tying up an economically important volume of seers, where their numbers come from - they're built, not born. It's why they have masks, so people won't see their terrible, misshapen faces! It's why King DJ can trust them in his scheme to bring an unknowable freakshow from the land beyond all lands into the world, possibly to destroy it - they don't have the will to disobey! THEY CAN'T BE MADE TO SEE THE TRUTH BECAUSE DJ CALLAN IS THE TRUTH.
Yes, yes, I like this fanfiction well.

I think I am still going to whine about being sold a bill of goods about how if they ever get spotted, they just need to "pop" the spell. Like, even a token thing! They try to pop the spell and the Battlecaster just grabs them by the magical genitals and says NO MAGIC DOESN'T WORK LIKE THAT YOU COWARDS YOU FOOLS. Sweat a second! Luna was wrong! And then Celesto's all "lol jk gents p.s. hack the planet"