We often confuse Free and Open Source with "free beer". We have been using pfsense for years without paying a dime; if we could we would gladly have done it. It's simple fairness.
There is this conventional wisdom that a lawyer is not really your lawyer until it is you who is paying them. I think it has much broader application and you want to be paying for technology. It is naive to think you can accept services of say Facebook, not pay a dime for it and assume it is all done in your best interests.
- software is copyable for free
- contributing to open source doesn’t make the developers your agents like paying a lawyer does
It does not matter if there is zero cost to copying the software.
What matters is that somebody put an effort into doing something and now they are seeing lots of people using it for free. It is in human nature to try to benefit from it and most people have a mental model that they deserve to be compensated for doing something beneficial to another.
Most people are not seeing incremental cost of adding another user (effectively zero or non zero), what they are seeing is total cost of producing the software divided by the number of users. Then they see that some users are paying and some users are not.