It isn't just Buc-ee's, it's common to all gas stations, and here in Arizona, not only is it common to leave your vehicle at the pump while shopping, it also isn't uncommon to leave the vehicle running while unoccupied! At first I thought it was to keep the airconditioner going due to the blistering summer heat, but it happens in the winter too when temps are in the 60s.
Yes, its a cultural thing, but it's also a result of a business decision to design a store, where the real profit is generated, without including sufficient parking. Since fuel isn't the primary profit generator for the business, the company doesn't care if the pumps are all blocked by shoppers. Fuel is the bait used to attract the customer. Once stopped, the retail sales are then end goal because that's where the real money is made.
When we full timed in our RV, I usually avoided places like this like the plague because they were absolute zoos. If pay at the pump didn't work for some reason, and I had to pay inside, I was often greeted with long lines at the register. After experiencing that nighmare a couple times, I decided, to vote with my feet and wallet. I never let my tank get low before topping off, so I don't have any problem pulling into a place, and pulling back out if it isn't going to work for me. I had the additional complication of towing a car behind the RV which preclude the ability to back up without unhooking the car. At one place, not only was the person inside shopping, they came back out to thier vehicle with a food container, climbed into the driver's seat, and start eating lunch at the pump. I was in the RV and couldn't back up to use another lane so my wife had to get out and ask the lady to move.