1. You're paying for the infrastructure that the support volunteers need to do their job.
2. You receive value for it. If the answers help save you time and money, do you care what motivates the people providing them?
3. You and the volunteers are both supporting a broader effort.
There are plenty of organizations that rely on volunteers that also charge user fees to support their mission; any museum with a docents program, park guides, ushers at the symphony, schools that rely on PTA volunteers to cover unfunded positions. It's not exactly radical.