In the real world, ot is increasingly rare to find employers who sign off on a daily rate of cash to be chucked at people for them to spend as they wish. Ultimately we are custodians of public money, we are obliged to try and spend it wisely and i dont think its unreasonable to claim actual expenses for what you actually spent.
Yes everyone has a story about an issue with receiepts, but this isnt a problem exclusive to the military and its just part of working life. At a time when military public image and awareness is declining thanks to the end of herrick, longing for the days of large chunks of cash just for going away to do your job, when everyone else has to provide reasonable receipts seems a good way to lose public sympathy pdq.