Put differently - the company are paying for the trip and for your time booking and going on the trip, why should they not be able to dictate how it is booked and paid for?
The worker's preference and the experience they are subjected to when they need to book travel matters, too.
Expense policy nonsense was regularly a point of contention between consultants and management when I was working in infosec. And it was always incredibly dumb, because these were all highly skilled people who could leave and get a job somewhere else within a week. Some of them
did get fed up with it and go. Things like "we're rejecting an expense for paid WiFi at the hotel with no mobile coverage we made you stay at for two weeks".
Clearly in some organisations you can afford to do "employer's approach or the highway" and the civil service is very much like that to a large extent, but in other orgs you need to actually listen and implement something that people are happy to use. And sometimes a flexible policy which allows people to use their favourite booking engine and retain personal cashback, but still requires them to furnish appropriate documentation for an audit trail is a good compromise.
