First of all, Google AdWords *can* be a great way to drive traffic and business, but it takes a lot of experimentation, A/B split testing, etc.
The whole point of spending that money is you want to get a return on your investment. So if you spend $50 on a campaign, you want to try to earn that back and more.
If you don't carefully keep track of what you're spending, you can be out of a lot of money very quickly.
Google knows that most people need to spend way more than what's on that voucher, so it's actually a very smart marketing strategy to dole these out.