Once a year, the normally heavily guarded main gate of Yokota Air Force Base in the distant suburbs of Tokyo is flung open to the Japanese public for a weekend event known as the Friendship Festival. While the open house primarily targets locals in the immediate vicinity of the base, one enterprising travel agency is marketing bus tours bringing in thousands of Japanese from as far afield as the opposite coast.

Other bases in Japan are expected to draw protests this summer, but Yokota's festival typically attracts an excess...