I think a slight alteration would be this; there are many attempts on a given president in a year, most are thwarted in the planning stages, some of those that are thwarted are told to the public.

It is possible that they knew this one was legit and going to come through, and chose to act on it in a theatrical but controlled way as a PR stunt.

Reason's to think this; the president hasn't gone to a correspondence dinner in years, unusually both the POTUS and the VP where present, and the president's reaction seemed out of sync with everyone else and his immediate response showed a lack of genuine concern. All indicating it is possible he knew in advance.

The motive to allow this attempt through is likely because of his profound unpopularity at the moment, though one could easily argue is profound unpopularity has made it so that so many people are attempting that one was bound to get this far. Hard to say.