It clearly says this on the machine in large print. Second, vending machines usually don't work this way. Typically, you first insert money and then you make a choice. If you try to insert money and the machine doesn't respond to it, the most likely reason is that the machine is out of order. I told another friend at the recital about this problem, and she said she couldn't figure out how to use it either.
People expect vending machines to work that way.
Printed instructions, even obvious ones, aren't going to be read by some people, especially people in a hurry. People skip reading operating instructions for devices they believe they already know how to use. They already know how to use the device.