>Why should I beta test your commercial product for free?
Curiosity to see how good is your C using a new metric? Fun? Try a test which you could encounter in a job interview?
>Isn't that something you would normally expect to pay for?
Standard software industry practice for external beta testers is that they do it either for free or they get "paid" in kind (e.g. a free product licence), but only if they provide meaningful feedback.
Anyhow, it is your choice whether to take part.