I think requiring payment is a bit too strict requirement to define a customer. Your users still agree to your license, so there is a relationship established, you may just not get any benefit from it (monetary or otherwise). Even in your license you likely have to be explicit that "software is provided as-is" and you aren't responsible for it misbehaving - otherwise your customers/users could try to sue you. Just because you don't pay for Ubuntu doesn't mean Canonical does not get anything out of you deploying it. Do they gather any kind of data about users' behavior?