I suspect the whichever department is responsible for the intersection has some internal rules about where they put what kind of traffic signage and this merging area is too small for a proper merge with a yield sign so they just slapped a stop sign on it knowing full well that most drivers will treat it like a yield it in light traffic conditions.

Of course this causes problems when dutifully law abiding drivers from out of town follow the law to the letter and cause near misses or rear endings because they are behaving unexpectedly compared to normal traffic but that's not the problem of the people who put the sign there. Ignoring these kinds of edge cases and expecting someone else (often the courts) to sort it out on a case by case bases when it causes problems isn't exactly uncommon in government. It's like their version of an unanswered bug report from 2010 with a bunch of "hey I found this by googling and I have the same issue" comments below it.