There is now text at the bottom of the widgets page saying that the weather data is provided by The Weather Channel. Maybe they're legally required to put this there but it's very ugly in my opinion. They should at least contain it within the weather widget itself. I like the design of the stock weather widget and the icons they use but that alone makes me kind of want to remove it.
That weather info text at the bottom (aside from iOS 10 beta 1) has been there for ages. iOS 9 has it and I'm quite sure iOS 8 and 7 did also.