Any such IDE needs a way to communicate with the services that are going to enforce the permissions. The DSL is a good way to do this. It allows inspection, diffing, and access by other tools, instead of being locked away in some hidden IDE binary structures.

Edit: But no IDEs will yet have any graphical tool to assist in rule development and checking. So the fact that one is provided, is better than it not existing at all. But you can always edit the DSL manually, if need be. They explicitly state that their rules-editor can sometimes be limited:

"The Rules Workbench, a visual rules editor that you can use to model most of these patterns"