I haven't been part of the rF team in years; however, I was around when the rule was created. It was created because there were riders in the top 3 in the 450 championship dropping down to hopefully sandbag a 250 regional title. The goal of rF was always to create a level of racing where each rider is competitive and the elite players race the elite series... 450 SX. Clearly you are an elite player if you are near the top of the series after more than a handful of rounds.
I'm not saying the rule is right or wrong or that 15th place is the magic number; however, that is why the rule was created and the thinking behind it.
While there isn't a rule in place IRL that if you're top 15 in points when East starts you can't drop down, there is a rule the top 15 in points from the previous year's 450 class may not drop down without petitioning the AMA (Chisholm has done it in the past and Friese did for this season for example).