This is an excellent question. I struggled with this when I was launching the StackExchange 1.0 version of the site. I never did settle on an answer, but here's how I was leaning:
Some limited types of questions that are small-business-related could be on-topic as "personal finance."
For instance, some questions related to being a contractor, self-employed, independent, freelancer, etc. (whatever you call it) ought to be on-topic, because they relate to a fundamental decision all individuals have to make: "How will I make a living?" Traditional employment is but one way to make a living, and to only allow people to talk about their employment income to the exclusion of other forms of income would be prejudicial. There are other forms of making a living and I think they can be explored. Even trust-fund brats could come and ask trust-fund brat questions :-)
On the other hand, I think if a questions relates more to the how-to-run-a-business side of things, or clearly to larger businesses (i.e. obviously not one-man-shops), then the question could be considered off-topic.
Of course, I have to confess a bias: I am an independent contractor, and not an employee. Consequently, I want to be able to talk about being self-employed, how it affects my finances, etc. :-D