That seems like it might be an overgenerous treatment of autonomous vehicles' (current) risk evaluation. The article reads like humans are making the determination that the vehicle can't drive in the rain, not the machines.
What I mean is that the autonomous vehicles probably just get very noisy signals in the rain. Isn't that also what happens to humans driving cars in heavy rain? The rain limits long-range visibility, fog limits short-range visibility, wet road increases stopping distance, and yet humans think it is safe to drive at regular speed limits. Perhaps humans are (for now) better equipped to deal with those very noisy signals, but my point is that it's also dangerous for humans to drive in heavy rain.
