Probably because the source is still open. This whole discussion strongly reminds me that open-source and free software are not the same.
This sort of confusion is exactly why this battle is worth fighting.
The proper term for licenses like the BSL is "source available". You can view the source, but you cannot use it under terms which conform to the Open Source Definition.
> It’s a brave new world, but we believe this licensing change will best ensure the future of Sentry and that protections-oriented OSS licenses like the BSL will become increasingly common as time goes on.