As others said, some may see it as a threat, others see it as you having passion for your craft. I believe the real question here is "what do I say to an employer if they are concerned about my side business/projects?"
In general, be honest, and be willing to explain how your side business/projects would affect your ability to perform for them as an employee.
I have my own company, and this had come up for me in an interview. I was asked how much money the company was making and how many hours per week I dedicated to the company.
In my situation, I explained that the company was incorporated for legal protection for my side-projects, and that my full time employment always comes first. They asked a few followup questions, and I answered honestly. In the end, it wasn't a concern and was offered the position.