People like having choice. The SSPL provides less choice than open source licenses. It came about because users were utilising their choice in way the project leaders didn't like, because they didn't directly financially benefit from it. But without that choice, arguably Redis would not have reached where it is today.

If it had launched as some proprietary single-vendor cloud service, people would have kept using memcached, or their relational DB or whatever for a lot of Redis use cases where it might be nice, but not essential improvements over the competition.

So it's shutting the door behind them, accusing the users of taking advantage of the very thing that let Redis get to where they are today. (Especially so for Redis, where Redis Labs started as just another hosted provider unassociated with the open source projects)