Said person should then search for BUF and ROC separately, or should have an opt-in action to check a box to include other airports.

If you assume uniform population distributions across the map - far more people will be inconvenienced searching BUF and having ROC results, than the small portion who would also consider ROC. Hence, it should require more action by the smaller portion of people being open to both airports.

If I'm at a grocery store and ask for Fuji apples, the store should not automatically assume I'd be open to Granny Smith as a replacement. They are both apples but quite different in texture and flavor.