Attribution of delays to weather can cover a lot of evils, even when done legitimately. That's because the effect of weather on an airline depends on how an airline configured its operations. Not all airlines are affected equally, for example if UA had a less tight schedule, there might not be a weather delay in those example cases. In all those cases, the flight delay is not because of weather alone, but due to the airline relying on a plane coming from somewhere else affected by weather.

But that's how it works -- an airline can't have a dedicated plane ready for each flight on its own, and allowing them to say that weather is the cause (even if operations is the cause) relieves them of having to do that very expensive and overkill solution.

Could they sub-categorize weather delays into pure single-event causation, versus cascade delays due to schedule/operations choice? It would be complicated. Consider how, for example, a weather problem in ORD wreaks havoc on UA, but means nothing to WN. Another example -- thunderstorms in NYC rarely delay JFK/EWR-->Europe-bound heavies, despite all the connecting flights being screwed, because the planes were already there (generally).

How about JetBlue's A320s constantly (in the past) having to make fuel stops halfway across the country when the winter headwinds kicked up? That's called a weather delay, but is that weather, or their operational choice of aircraft?

And for example, the much-grumbled-about takeoff/landing delays at congested airports. It's fair to say that weather delayed the first plane after the weather passed through. Every other delay after that first plane is more legitimately understood to be a result of how the airport/airlines choose to pack the schedule. If you have a private jet and can take off from other airports, there is no such thing as a weather delay!

I've come to understand that weather does not mean that weather prevents any flight from taking off. It means that weather has affected how the particular airline chose to operate. And that is what you live with in your choice of airline!