Flying Vending Machines

Consumerist has a post calling US Airways Planes a "Flying Vending Machine". This reminds me of a recent experience I had with Northwest.
The wife and I were flying back from California last week. On northwest they no longer give out complimentary "fun size" bags of pretzels nor do they provide any type of complimentary food. I'm cool with that. What they do offer is a variety of food options for a price. One of the options, the one we chose was a $3 bag of trail mix. Delicious. It was very good. Exactly what we needed.
Why would I write about this? Well I only had a $20 and the flight attendant didn't have change. So she took the $20 and said she'd come back. She went up the aisle and back and did another trip to pick up cups. Then she did a coffee trip. When she asked me if I'd care for coffee I said yes. I asked her if she had a chance to get change she said that she hadn't.
She came back to me and explained that she didn't have change. Was I going to have to pay $20 for a bag of trail mix? I asked what we could do to take care of this?
She asked if I wanted anything else from their snack menu and I was thinking, NO.
There was a happy ending though.
A very nice man from across the aisle was kind enough to make change for a $20 and I was able to get my $17 in change. If he hadn't been so kind I would have been out some money. What's wrong with this?
If airlines are going to start acting like merchants by charging their passengers money for things, shouldn't they take the same responsibility that other merchants take and provide a way to make change. I sincerely believe that I may have been out an extra $17 had other passengers not come to my aid. I know the airlines are struggling, but that's no excuse for trying to pull nickle and dime short changes on their customers.