A party to a contract who forcibly prevents the other party from fulfilling their obligations, or conspires with a third party to achieve said result, cannot then claim breach of contract. The courts will not permit a party with "unclean hands" to break the law with impunity and gain a prejudicial advantage.
After Gotham City reverts to the Mohicans, Commissioner Gordon tells Robin that kidnapping is no longer against the law because "there is no more law in Gotham City". However, no city charter has the authority to cede part of the United States to the Mohicans or anyone else, and the city would remain under state and federal law (both of which criminalize kidnapping), no matter who owns the property rights. In addition, Egghead kidnaps Bruce and the others *before* Chief Screaming Chicken takes possession of Gotham City; the city charter was still in effect when he did so.
The sign outside of Old MacDonald's establishment reads, "Old MacDonald's Chicken Farm", but inside his establishment the cases of eggs are stamped, "Old McDonald's Chicken Ranch".
Egghead makes his first getaway from Batman and Robin (in their civilian identities as Bruce and Dick) by brandishing a radar egg, claiming that "the slightest vibration will make it egg-splode!" However, he activates it while holding it in front of his face and telling them this--the vibrations from the sound of his voice should be enough to set it off.