I imagine this is here because of code reuse - but since you can only select one release here to base your new one on, the checkbox is just confusing and does nothing.
In review at https://github.com/metabrainz/musicbrainz-server/pull/22
This is causing an issue in beta – in the RE the "artist" checkbox (on the tracklist tab) isn't appearing because this hides it based on the fact there are no checkboxes to use (because, in the RE, they're added after this code), and doesn't enable it when checkboxes are added.
Oh fuss. I'll add this back today then.
I add a link to MBS-3629 in case it's the same check-all checkbox macro that is missing in subscriptions.