I think we should not place too many limits on the scope. If the question is related to WordPress, I welcome it. I use the same reasoning as in the WordPress.com vs .org discussion: if we are too strict, other sites will pick up these questions, and start answering real .org questions too.
Yes, these premium themes and plugins usually have support areas of their own, but WordPress.org also has a support forum, and that's not a reason to close this site, or is it?
Of course, the probability that someone can answer a question about code that is not freely available is lower, but not equal to zero. Stack Overflow also has lots of questions about proprietary technologies that get answers.