If branch being reviewed by a pull request happens to be closed and merged back to the target branch outside of BitBucket's Pull Request, the only way to close the Pull Request is to "decline" it. This makes it look like the change was rejected, when in fact it was accepted.
Most of the time, our devs want to just use the "merge" button for a Pull Request to merge back to the target branch, but we can't guarantee that's always the case. For example if a there are conflicts in the merge to the target branch, they'll have to be resolved locally and the user may want to just handle the merge back to the target branch at the same time. Having to mark the pull request as "declined" in this case leaves a false trail of history that makes it look like the change should have been rejected rather than accepted.