Seriously though, I'm an indie iOS developer with a semi-popular app. I have hundreds of TestFlight users, but I only get feedback on betas from a handful.
This can be frustrating, considering the whole point is to get things right for many edge cases before it leaves beta.
(Hence my defense of Planet having TOS requiring feedback for their beta).
Many companies do this to test the robustness of their system by letting people in in controlled beta-invites waves instead of going 0 to 60 in a stampede inauguration. The tight loop you're referring to seems to me more typical of QA/QC testing, which is a paying job. Trying to pass this job into customers is tricky, since most of them are not interested in following tight reporting schedules or writing lengthy standardized reports.