The moral equivalent of the BSL is to close-source the software, and to offer free-for-use licenses of the closed-source version. They can continue having a fully open-source release, but the release would be changes from 3 years ago.
Sentry isn't doing this because they want the contributions. They want the benefits of open source, without any of the costs associated with it.
Sentry is granting you a license, for free, to do anything you want with the software except provide a commercial competitor to their hosted service. People using the software for free may still decide to contribute code. This is not a one-sided relationship here. Both parties are getting something of value.
If the contributor is not allowed to do something with their work that the receiver is allowed to, is that not one-sided?
The benefit you speak of is available to both parties. The benefit I speak of is available to only one.
In Open Source, every party gets all the benefits, all the time.
Also, the closed-source code not being viewable makes it more likely that folks won't accidentally introduce copyright violations into their code, which frequently happens when proprietary code is put into github.
In either case, the code is closed-source, but BSL allows them to accept free code from volunteers without providing it back to them for 3 years.
You're suggesting a strictly worse outcome for the majority of people (users, and sentry) in the name of purity.