I can see both sides of the issue. If it is a routine question easily and quickly answered, no charge. If it requires any sort of medical effort or research, then I could see a nominal charge. But the answer there could just be make an appointment.
For instance, MFR just got a prescription for a drug where she is contraindicated by another condition. There shouldn't be any charge for that question. It's a clarification of an order.
As a relatively frequent user of MyChart to communicate with my docs' offices, I really don't want to see a charge. And George is right - 95% of the time the SP or PA answers the question.