@Background Pony #314F
 
Depending on how one interprets the episode, it might be that the reason only Ponyville pegasi were working on the tornado was not necessarily that only they were allowed to do so, but that it was more of a matter of hometown pride. A big part of the start of Rainbow’s speech was that she wanted to beat the wingpower record Fillydelphia had set when it was their turn to provide the water, so beating another town because they had the Wonderbolts to help would probably make the victory hollow. Also, given how the water is used to provide weather for all of Equestria, it wouldn’t really be a good idea to deprive the entire country of rain, snow and clouds because what is strictly speaking a very arbitrary requirement couldn’t be met.
 
The point I’m getting at is that while it may that only having a given town’s citizenry work to provide the wingpower is perhaps something of a tradition, it is still likely that if they can’t manage by themselves outside help can still be called in.
 
But to go back to the hometown pride argument, then even if the town can meet the base requirements without them, Rainbow Dash and Thunderlane are still Ponyvillians – calling in on two of your hometown’s most successful sons and daughters isn’t quite the same as begging the government to bail you out, insofar as pride goes. A victory won thanks to those two would still be a victory for Ponyville, whereas one won thanks to the entire Wonderbolts team… wouldn’t really be.

Now that Rainbow Dash and Thunderlane are both official members of the Wonderbolts, what would happen when the Ponyville population gets picked to bring the water up to Cloudsdale again? Would the two’s status keep them from being able to participate with the other Pegasi, being considered an unfair advantage? Or would it still be okay for them to fly with the others?