Some projects require signing copyright transfer before making commits (legal document claiming that you are a) copyright holder and b) you transfer those rights to them ie CLA [0]) so single entity holds whole copyrights.
They usually have a GHA that checks it when proposing PRs.
It doesn't look like redis has any of this.
So they run RedisLabs purely on trademark + admin rights on GH on redis/redis.
If that's the case then they also cannot legally change licence of code that's already there because they're not sole copyright holders of that code.
ps. as a side note that's why ie. SQLite doesn't allow external contributions at all, even though their code is Public Domain – because they can legally claim full copyright/authorship.
[0] https://en.wikipedia.org/wiki/Contributor_License_Agreement
Public domain isn't the only form of free software. You can literally use it in exactly the same way as you did before. Nothing has been taken away from you.
Does this address your concern?
One of the well known flaws of capitalism, in the 'bad, but everything else is worse' sense.