There's your answer. If there's nowhere to park, the remaining option is to park in front of the place of delivery while the delivery is being made. That's happened in cities I've lived and visited for as long as I can remember.

It may be inconvenient, but I'm not sure what other options they have in some cases.

Usually there is a driveway with room to park right beside the vehicle parked in the road. It doesn't take much time to pull in, deliver, pull out. I don't have a beef with the situation you describe.
A few weeks ago, a UPS van parked in such a way that if I had tried to go around, I would've hit a mailbox on the other side.
I don't think drivers in motion should be blocked.

Amazon dude said they can't even use the driveway to turn around.
My but-but safer has gotten me no where. I try not to have anything going on trash pick-up days.

… some of them turn around in our dive, not an easy thing with a delivery van as the drive is narrow and bordered by deep culvert on both sides. Like the FedEx driver who parked in our dive for several minutes yesterday. (I wondered if he was napping…)

Delivery vans do often park in the road here, too. But not always.