Because they cannot get a real credit card, and "sometimes" various employers or "services" issue a form of prepaid credit card which they deposit payroll or "monthly" allotments into.
But once again, they could use "Those" cards to buy "These". I've been using them for a long time... Whenever friends of family want to gift me something (xmas, bday...) I ask for these and use them to pay for my service. If I don't have any I buy a yearly and pay for it all at once. And yeah, you do still need a credit card, but my debit card works just fine.
Go get a bank account...I've got some bad points on my credit history but it never stopped me from opening a checking account. Seriously banks are everywhere ( even in Walmart for pete's sake ) and many don't ask for a minimum deposit or a monthly fee. As soon as you get your debit card in the mail you won't have to worry about how you'll pay for online services.
It's how Netflix is making sure their product gets back to them. It'd be easy to purchase a gift card and never return the disk(s) after your paid period is up. This way they can charge the credit card on file accordingly if such incidents occur.