Their explanation is a lie
They claim it is "part of an experiment to optimize the routing of messages through mobile networks", but a phone's OS has NOTHING to do with how cellular traffic is routed. That's entirely up to the carrier.
I'm with the guy earlier in this thread who suggested it is/was related to targeted advertising. They want to know where you are to better target ads at you, and location services being off was getting in the way of Google's profit. Maybe they weren't saving the information, since it would only be needed at the moment to deliver the best ads to you (i.e. if you are near ice cream shop A, you get their ad, whereas if you are near coffeeshop B you get their ad) but it still is against the user's wishes if they've disabled location services.