I think all the boarding in the US is done by ground staff as with the 1:50 F/A to pax ratio there would only be 3 F/A's on a 738 who all need to be onboard to oversee the carry on debacle.
But I agree, they do have a 'take no prisoners' attitude to not just boarding but other aspects of customer service which we may not necessarily want to see on our shores.
In Australia it's a CASA requirement on 738's that three cabin crew members must be on the aircraft at all times during the boarding process.
There are 4 F/A's on a QF 738, CSM + 3 others. From what I've seen it's the FA that does the safety briefing for the exit row pax and stands at the front of the Y cabin doing the general safety demo that does the scanning at the gate, they may or may not be later on joined by ground crew. Of course this is only what I've seen and could be way off the general mark.