Who do they have to pay. I would assume they can get updates like the default does. My issues with the default is they break up separate items; temp, conditions, wind, rain..... I really like the MyRadar complication on the infograph face cause it has temp lows/highs, colored temp graph, current temp AND conditions. They just don't update in real time unless you open the app on the watch.Carrot updates the data on its complications frequently, but you have to buy a subscription for that.
Weather app developers have to pay for every weather data update their application requests. In case of Apple Watch app, this can mean for example 10-20 updates per day if you want the complication to show up-to-date information.