Not sure where you got this information from but it makes no sense and is outright crap.
My friend had an outstanding balance and three accounts all on the same credit card. No one could make a purchase until the debt was cleared up. You say itunes is low profits for apple? PC and Mac users buy from itunes and your account with itunes is the app store too; those combined make more money than apple hardware that is sold annually--check the markets and you'll see.
Basically, what happned was he paid the debt on the one account with a gift card then everyone could use the card again. Also, he found out that the card was rejected because his address on the account in itunes changed and he never matched it to his new bank records.
Just like every other sales place, when you make a purchase, they ping your credit card's servers to see that your card is alive and has enough money on it to purchase stuff, in the terms it says for an amount up to the cost of your purchse or a bit higher. You get to download, then your credit card company takes it's time and pays apple when thier system puts the payment through on thier end. If I have 75 bucks left on my Visa, spend 25 in itunes, then go buy food for 25 I won't have enough money when Visa gets around to trying to pay itunes because there will be a test in the amont of 25 or itunes, and a test for food for 25, plus the pending payments in the amount of 25 and 25...... that equals 100 when I only have 75. Now my card is rejected because Visa knows I don't have 100. Until the holds clear, I am declined. That can take a few days.
If my address is wrong, like my friend's was, then nothing gets paid to iTunes. How can your card exsist if your address doesn't match? You live nowhere with no place to send a bill to.....
Also, just because you think that the amount is small and that is why they won't come after you is pretty niave. If there are a 50 thousand people that all owe 30 dollars each that's a lot of $$ and it's called "class action". They already have lawyers they pay all the time, so why not nail people for what they owe?
AND.... it's just rude to buy things and not pay for them in the end. Even if it was an accident, you should still pay, then ask for a refund. It's kind of like eating an entire meal in dinner then saying you never meant to eat it. It was selected, then brought to you, then consumed.
Do you know what resrtictions are on your phone or whatever--they help prevent accidents. My nephew uses my phone to play games, so I set those up and it's all good now. http://support.apple.com/kb/HT4213
Anway, it sounds confusing but makes sense so if you are confused about tests and holds then talk to your bank.