I don't think that's the case. The US doesn't recognise 'transit' - they would have to enter the US which involves reclaiming their bags and clearing US customs before rechecking them for the connection.

They may be able to drop them off right after customs without having to go all around to landside checkin, but last time I transited through MIA, I had to take mine all the way out and join the checkin queues.

I was rather surprised by this and asked about half a dozen agents and they all told me the same. There is no such thing as transit.