For most folks, it's easier (and perhaps cheaper) to change a sign than to deal with a lawyer.It isn't. The relevant legalities here would involve trademark, but I don't believe "Aikido" is trademarked in the U.S.. Perhaps Mr. Toyota was able to intimidate dojo owners who weren't familiar with trademark law.
Given that the Dai-Nippon Buttoku-kai designated "Aikido" as a name for arts of that type, I can't imagine it would have been easy to pursue a trademark case. More likely, he was challenging their use of the name to claim something they didn't have (either a link to their hombu, or rank to teach).