Yeah it's annoying, I used to get really frustrated with it as well. I think they do it to get people hooked. At low levels where money is precious, you either have to play for hours to make up the cash, spend real money or stop playing completely. It won't work for everyone but I bet it does for people with more spare time.
Once you get to higher levels you have more money than you can spend, so it's not an issue. But the game economics in the beginning are rubbish. Looking back, I can't believe I stuck with it. I must have had a lot of time on my hands!