I see this constantly in my industry. Mid-level managers (and above) are always trying to max travel miles on company expense. If I ran my own company (dream!), I would never allow employees to reap "travel miles" from travel paid for by the company. It is ridiculous.
I've always understood it as a perk the company knowingly allows its employees who need to travel.
Are you going to ask your employees for their 2% cashback if you reimburse them for a purchase they made with their credit card too?
At the end of the day, it seems more practical to focus on ensuring that they picked the appropriate value product/service. If they picked appropriately, does it matter if the company gives them employee a kickback? Sure, the employee will then be incentivized to get the most expensive thing they can, but this was already the case because it's not their money and they want the best thing they can get away with.
The company isn’t using the travel miles. You’re basically saying, “I’ll take away a small perk from people who are forced to travel.” It’s not like they’re stealing from the company, the company sent them somewhere else and they just got a tiny bonus for it.
The proper thing to so is stop those managers who travel too much.