Mr. Rewrite is from Arizona, so from painful experience he knows his way around a cactus. He also knows that AP style is cactuses rather than cacti when referring to more than one cactus.
Outside of stories involving abortion, water policy and endangered species status, few topics have prompted reader feedback as acid as those requiring the use of cactuses. Some readers go nuts over it.
All Mr. Rewrite does in such cases -- after calling his mother in tears -- is send the prickly reader the AP Stylebook entry and tell him or her to contact the world's oldest and largest newsgathering organization with any concerns.
Cactuses as AP style probably has outlived its usefulness. But as long as it's in the stylebook Mr. Rewrite is going to follow it blindly and be happy to do so.