I'd imagine if enough people got to thinking about it and talking about it and supported a culture against solo bin scanning it could be eliminated eventually. This, to me, would have to start by reaching out to the leads and deciding how best to do the job while avoiding having one person alone with the scanner. The easiest solution may simply be to unload one bin at a time, and if the local bags take a hit on the time that's just too bad. If leads know that other leads are willing to do the same and back them up it will be easier to stand up to a management obsessed with metrics.
Probably because it's how we were shown to do it since the beginning. Before they rolled out all the scanners they had one gate use them for a couple of months beforehand to test it out, and I remember working that gate at the time and we were scanning in the bin not knowing any better. This was a gate that at the time could not accommodate 321's or 757's and so the scanners weren't tested on those bins. Being at the bottom of the belt loader and scanning bags as they come off and tossing them into a cart isn't very ergonomic or easy either.