With married sector availability, with a normal ticket, if booked A-B-C and then wish to drop A-B, then

if this is not a flight pair where married sectors are set up,, then no issue
If it is a flight pair where married sector availability is used, then B-C will rebook into an available class and any change fee and fare difference will apply

Being able to drop a sector without incurring the change fee is the benefit that AA offers ; I don't see that there is anything stated that the fee exemption for dropping a sector was "irrespective of award availability "

For some , married sectors are a benefit, for others a drawback. It can mean that those needing to connect now have a better chance of getting a desired award

It allows an airline to assist those needing to book connections by segmenting availability between those that just booking hub-hub vs those needing to connect to get to a destination; instead of , say, 4 flights from hub-hub, it can be split between those wanting hub-hub and those needing to connect

On paid tickets, it is a way of limiting availability from cheap routes vs more expensive. Have seen on QR a lot of times where A-B and B-C have been available for a discount business ticket whilst a very cheap A-C via B has been unavailable.