Underfilled pooling is not a great example though-- filling in load shortfall on pooling is a very efficient way to spend money, because you need to build the load, uber needs to only profit on average, and even if there was no subsidy at all there would occasionally be underfilled cars.

For all you know they did five other trips like yours but with three passengers and your "subsidy" was paid by them; and in exchange uber got a happy passenger and driver which will continue to support their business in the future.