No they haven't. "The community" submitted under a BSD licence, they literally gave anyone permission to take their code and relicence it under an alternative.
It's not like the BSD version of their code has disappeared. You can still use older versions of Redis that have their changes under BSD.
It sounds like submitters should have gone with GPL or AGPL if they wanted it to remain open source, available with the original licence for commercial use, etc.
This would only be immoral if you were misled or forced, and I'd argue that neither is the case here.
All the contributors gave their explicit permission to use their contribution in the way Redis does.
All contributors had plenty of freedom to spend their energy in projects under a license that specifically prevents what happened with Redis. It is not that they wouldn't have other options.
What they did do is decide that in the future, their work won't be as easily taken advantage of.
Legally, sure. That's pretty binary.
But if you claim they have the moral right to do so you need to elaborate on that. Since they had a "social contract" with the community (people who submitted PR's, advocated for Redis, etc.) which a single side has now altered. I don't see how one can do that an claim to be in their moral rights to do so.
We've altered the deal, pray we don't alter it any further...