Well, if they are transmitting, at least in theory it would be possible to detect that transmission. Though I would be kind of surprised if they actually put much effort into actually enforcing it.
The law (it’s short): https://www.law.cornell.edu/cfr/text/14/91.21
An advisory circular relating to it: https://www.faa.gov/documentLibrary/media/Advisory_Circular/...
This allows an air carrier to approve anything they’re confident won’t cause an issue but not approve AirTags.