I thought the visa requirement was based on distance of travel into Mexico. First 100 kilometers (62 miles) doesn't require a visa, and Rocky Point is a borderline case that Mexico allows.
No, distance has to do with the vehicle temporary permits which are enforced when driving approximately 20 kilometers into Mexico or, in Sonora, after the hassle-free zone ends. In addition, enter Mexico with valid proof of automobile registration, even if remaining in the border zone. Entering Mexico with an expired U.S. vehicle registration may lead to the confiscation of the auto by Mexican authorities.
"Officially", U.S. citizens must present a valid U.S. passport book or card, in addition to an entry permit (Forma Migratoria Multiple or FMM) issued by Instituto Nacional de Migración (INM). Again, not usually enforced at Sonoyta crossing.