I'd put the blame on the rather silly design of the stand. It's most likely the owner of the other bike genuinely thought they were locking it to the stand. That obviously isn't always the case in these incidents.
I don't think it's particularly silly: A variation on the classic Sheffield design, which provides more locking point options for bikes without a traditional diamond frame that might otherwise end up sticking out or badly supported in order to be properly secured. It likely makes the stand easier to detect with a cane
[1], as a side-effect (normally a horizontal plate is fitted to the stands at the ends of a row to achieve this, but the 'M' design is probably easier to manufacture).
Ultimately, locking up a bike securely is one of those visual-spacial tasks that some people are naturally good at, and others have to learn step-by-step. It probably correlates with ability to draw a bicycle. If you've had to learn it as a sequence of steps, you're going to have more difficulty adapting to unusual stand designs, etc. That the OP's bike has a toptube that looks a bit like a bit of bike rack doesn't help.
I'm leaning towards the mistake theory, rather than the secure-as-precursor-to-theft, simply because there's nothing to stop the OP coming along with a suitable vehicle and carting off both bikes.
[1] A traditional Sheffield stand appears to be a pair of posts until you walk into it.