It's not a regulation. It is conjecture. Ford doesn't offer the Bronco from the factory with a soft top because they didn't think there were enough vehicles that would be configured that way to warrant it. They could offer it with greater profit, and less chance of loss (vehicles sitting on the lot longer) as an accessory. That way they don't have to worry about configuring vehicles for the lot that wouldn't sell.
I recently ordered a 2021 Jeep Wrangler two door with a soft top. My salesman has worked at that Jeep dealership for ~22 years. It was the first two-door soft top JL he'd sold, so since before 2018 (when the JL came out). They don't order them that way for the lot, it has to be a customer order to get a two-door soft top.
2022 Jeep JLR, manual with even more extra guacamole
Sold - 2021 Jeep JLR, manual w/extra guacamole