>> if they do find something, you'll have some security guards come up to you and escort you to a separate section of the airport

I'm saying I have dozens of personal counter-examples to this claim, including another one from this morning, specifically relating to travelling with checked luggage containing batteries.

Hypothesis #1: maybe the "no batteries" policy is nowhere near as strictly enforced as people think it is?

Hypothesis #2: maybe other aspects of aviation security are nowhere near as strictly enforced as people think they are?

Who knows, maybe it is mostly security theatre?

Or maybe you didn't get spotted so far and the airline you travel with is lax in this regard. I've definitely been pulled out for batteries in the past.

Q: is it really "the airline" or - more likely - the [relevant] government a) setting and b) enforcing policies such as these?

Usually a mix of both, depends heavily on where you are, in my country security is mandated by government but the exact procedures are left to the individual airports and evaluated by government agencies.

IIRC the US has the TSA but Airports can outsource to other third parties if the TSA permits it.

I don't see how that question is really relevant considering your bag will be atleast xrayed by one of the two at some point.