They don't get to choose. They don't own the copyright on Redis, Salvatore does. He is their employee now but if they try to pull something like this I suspect he won't be for long.
Based on this interview from 2016 Salvatore
agrees with the issues of BSD and cloud vendors so he may be a lot more amenable to a license change than you suspect: https://venturebeat.com/2016/06/19/redis-creator/
>But now, for the first time, GPL could be interesting again because of the cloud vendors. Because with BSD, cloud vendors are able to extract a lot of value from an open-source project, to the point of making it very hard for the project’s initial creators to make a business out of it. Let’s call it the “AWS problem.” The AWS problem, technically, could be enough in some way to create problems for the whole open-source ecosystem. Now people know that if they start an open-source project and put a lot of effort into it, they could be marginalized by AWS. That could mean that GPL would return again as the primary license for open-source software in the future.
