Well, first off Facebook API questions are still on-topic on SO, so they will have more eyeballs and therefore a greater probability of being answered.
Second, programmers can keep their existing reputation, while a new SE site would put everyone back to 100. Since the topic is a subset, keeping it makes sense.
I don't really see what problems this solution brings that can't be solved with a custom logo.
I agree with your second point, but your first point goes against established precedent at Stackexchange.
I mean, if it was about getting more eyeballs then I think the best way to go would have been to use tagging, so that questions would be posted to 'stackexchange' and would then be tagged with 'programming', 'codereview', 'superuser' etc and as they were tagged they would show up on the programming.stackexchange.com (and other) sites. IIUC the current way is that if I post a programming question to stackoverflow, it will get 'migrated' to programming.stackexchange. That works, but it detracts from the total number of eyes because it is removed from stackoverflow.
That design decision is something that has always kind of confused me about the Stackexchange sites.
It's about the topic of the site. FB API questions are on-topic with SO, others aren't. The Code Review is debatable since it's arguably a subset of SO, but other SE sites (like Programmers) don't really intersect that much.
Second, programmers can keep their existing reputation, while a new SE site would put everyone back to 100. Since the topic is a subset, keeping it makes sense.
I don't really see what problems this solution brings that can't be solved with a custom logo.