Originally Posted by dmodemd
...I swear I have seen others with more alternate airports including BWI in the WAS region. I guess you need to look at your fine print...
No, the other pairs that you've seen are co-terminals in the combinability section for fare construction purposes.
All current fare rules (and the standby profile) on UA only allow standby to other airports for those three pairs (LGA/JFK, ORD/MDW, IAD/DCA); specific fare rules may override this blanket rule.