A mailing list user reported that the "Enable stateful request payload inspection" option may not be working. Instead, he reports that the option is ALWAYS enabled regardless of whether that option is enabled in the UI. Here is the latest from the mailing list thread:

Thanks for your helpful information.
I can get request body with IPolicyContext object.
Besides, you also mentioned enable this on a per-API basis in the apiman UI.
Is It "Enable stateful request payload insepction" in API implementation tab?
I'm wondering because I didn't enable this option but can still get the request body in doApply() with IPolicyContext.
What is the difference if I enable this option?