Again, the local police will have the ability to investigate this to their heart's content. WB's decision to have the show go forward is completely separate from that. All WB is saying is they looked into it and think they're on firm ground to not be criminally or legally liable.

If they thought for a second that they might be facing some sort of criminal or legal exposure, they'd have canned dude and maybe the whole season (which is why they suspended production immediately).

Edited to add: believing her out of the gate is the safer, more conservative route from the perspective of the show. Worst case scenario is he raped her and they will face some sort of huge lawsuit from her and/or someone on the staff will be criminally charged. The best case scenario, as far as their pockets are concerned, is that she lied and no rape occurred. Corporate lawyers (should and usually do) have pessimistic, worst-case-scenario outlooks wrt high-risk situations.