For clarification: MongoDB is still Free as in beer, but not free as in Libre. IOW, Anyone can use MongoDB without paying money, but they cannot offer it to others as-a-service without providing the entire stack of hosting software back to the community. And of course AWS won't do that. So therefore the license is not 'free' to AWS, Azure, GCP, etc.
Redis is still FOSS under the BSD license. If Redis Labs were to change the Redis license, AWS would fork it in a heartbeat. To help clear up confusion you can read about the Redis, Redis Modules and Redis Enterprise licenses here: https://redislabs.com/legal/licenses/
While it is true that Salvatore retired last year, Redis is still FOSS under the BSD license, and it has a new governance team. Redis Labs is in the drivers seat with 3 of the 5 members, including 2 Project Leads. But there are members from Alibaba and AWS https://redis.io/topics/governance
If Redis Labs were to do something stupid with the Redis OSS license, Alibaba and AWS would fork it faster than you can HyperLogLog.
When used as a cache, Redis is often used as a 'look aside cache' (vs 'look thru cache'). Generally speaking, Redis does not talk directly to your database. Instead, your code looks to Redis, and then if it doesn't find what it's looking for, your code looks to your SQL database
Yes, but not Redis open source, which has been and always will be Free Open Source Software http://antirez.com/news/120. The 'Redis' that people pay for is Redis Enterprise, or any of the cloud versions of Redis such as Redis Cloud.
For clarification, here's the official response from the Redis Labs's website: "the license for open source Redis was never changed. It is BSD and will always remain BSD." Only Redis add-on modules developed by Redis Labs have changed from AGPL to Apache v2.0 modified with Commons Clause. https://redislabs.com/blog/redis-license-bsd-will-remain-bsd