Why an app have to cost 2.99$?

I have uploaded a little application to the Ubuntu Software Center. This time, I wanted set a price, I was thinking in 0.5$… but the USC has this requirement:

USC (minimum price app)

I really don't understand this minimum price. I think that the USC should learn about the Apple store and Android store: Small useful applications, with small prices.
Please, think on it ;)