Facebook will do whatever it can to protect itself from losing its users. Which represent Nearly $10B a quarter in advertising revenue. Sure the advertisers are paying that bill but without users to click the ad, Facebook would not have the revenue. And Facebook is much more interested in protecting the user experience than it is in your ad dollars. How To Set Up Your Facebook Pixel For Beginners In 2019 | Easy Facebook Pixel Tutorial
Now there is one other thing worth mentioning here. There is an optional section where you can set a limit for the campaign. How much you spend is usually managed in the next section at the Ad Set level. However if you know with certainty that you have exactly $100 to spend on this campaign and you don’t want to spend any more. You can set that amount here. To set a limit at the campaign level the minimum amount is $100.