Off the top of my head: 1. Charging then creates an expectation that doesn't otherwise exist. That may not be something you can or want to commit to; 2. There

Off the top of my head: 1. Charging then creates an expectation that doesn't otherwise exist. That may not be something you can or want to commit to; 2. There is an overhead in charging. Payments infrstructure, billing and so on. This isn't "free"; 3. Earning an income from your side project may well violate your visa status (particularly appropriate for those on US temporary work visas); 4. A side income may violate your employment conditions; and 5. Contributors may be less willing to volunteer their time to a paid product.