Winuser said:
I would consider being a beta tester if the requirement was you had to own a retail copy of the program to qualify. What I wouldn't do is pay for a beta copy so I could be a tester.
As many betas/technical previews I've participated in, Winuser, I've never been required to pay for a beta of anything. And, until the O365 2016 Technical Preview, I have never been required to own a retail copy/subscription of anything I tested.

I'm not sure why we were required to own a subscription other than that it would have been a logistical nightmare at the time the Preview was started. Or they could have wanted only those who were experienced with subscriptions. Or . . . any of a dozen reasons.

However, when the Preview was opened to the public, new testers got a six month trial, which wasn't available at the beginning of the Preview. So, it may be that owning a subscription could be waved in future. Only the Shadow knows . . .