I've read the rules. "Don't attack the enemy's uncappable base (main base). If they fire out you can return fire." Nowhere does it specify this is okay if the enemy team hold all flags.
It's kind of a given that if you have no where else to spawn besides uncap, it's fair game. The enemies aren't going to just let you run out of spawn to get to a flag lol
I get the confusion but no flags = uncap is fair game