It has to do with the way the line is set up. The person from the standby line has to actually swap sides with the person from the single rider line at some point and it's awkward, especially if you don't realize that it's coming and you're trying to swap at the last minute and you don't want to seem confrontational.
It's hard to describe, so I just drew up an approximation of the layout to visualize it. (I drew what happens naturally, if they don't swap sides, which puts the SR person closer to the ride vehicle at the end.) When you're in the hallway right after the two lines meet, there's nothing telling you which side to stand on, or even mentioning the two types of seats until the end of the hall. I really wish they at least had a line down the middle of that hall with markings showing which side is which - then they would send the standby person first, they would choose their side (typically the motorcycle, but I some people might actually choose the sidecar), then the single rider could be sent after them to fill in whichever side they didn't pick. Instead, right now they're paired up immediately and you just naturally end up standing with the single rider on the motorcycle side unless you intentionally switch.
There's also nothing outright stating in the SR line that single riders will likely be in the sidecar or that they get the second choice, so it's just kind of awkward establishing that the person from the standby line gets to step in front at some point.
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