It's acceptable because it's all they need and all they want. Anything larger than a 40qt tub (for my guys) and they get stressed and go off feed.
1. They need short enclosures because they are not arboreal snakes, they can hurt themselves when provided the chance to climb up high in an escape attempt. The ONLY time you will see a ball climbing is if they are uncomfortable. 2. They like tight spaces, not wide open tanks.
3. Opaque tubs are great because the snake can't see out, whereas a tank is completely see-through.
4. Tubs are great for humidity and heat. It IS possible to keep tanks just right but it's a lot of wasted effort IMO.
I only have two BPs, but I wouldn't ever put either one of them in a tank.