Omg, I still can't believe that was in any doubt. If you are required to be at work, then you should be paid for that time. Just because you aren't "working" does change that the business is requiring you to be there. If nothing else this should push them to make leaving work not take 20- 45 minutes. WTF. How does anyone think it's acceptable to [funcitonally] detain people.
From the business perspective, provided they can get a way with it, it is just another business decision, namely reducing cost and increasing profit. I would do the same.