The joke, at least on this side of the pond, is that the Americans thought they were buying Tower Bridge.

The London Bridge at Lake Havasu is the New London Bridge, which has been replaced by the Modern London Bridge. Some pedestrian alcoves from the Old London Bridge were rescued, and are in Victoria Park and at King's College's Guy's Campus.

From parent's linked article:

> Contrary to popular belief, McCulloch was not under the impression that he was purchasing the Tower Bridge of London.

Hence the term "joke" in the gp.