It also depends on your legal system and how much discretion it allows managers. Let me give you an example: my startup used to allow people to expense a car rental when travelling to HQ in the Bay Area, which was in general cheaper than Ubering everywhere (a daily Corolla is cheaper than two Uber rides per day unless the hotel is extremely close to the office). Some people decided to abuse that, fly in on a Saturday, rent a rather expensive car, drive the whole day around, then expense all that.

After someone noticed this was happening, the CEO decided to forbid expensing car rentals, instead of having to come up with a very detailed policy. The thing is that in my home country, the abusers' managers would have been allowed to simply deny the reimbursement on discretion, on grounds that those joy rides were unreasonable, which would easily hold in court (where a plaintiff would be automatically forced to pay the defender's attorney fees too). But due to the highly litigious US legal system, a CEO can't allow that and companies must have extremely detailed policies, otherwise they risk very costly lawsuits where even if they win they'd lose a lot of money.