Why? Because you don’t even need a user name and password to buy stuff with, just a simple button click. You’re already signed in and they have your credit card details since you have to enter all that guff when signing up.
The annoying thing with Marketplace is that you can’t buy things directly with your hard earned cash, you have to buy Microsoft Points instead. It’s kind of like buying credit for your account, but it’s a bit of a mess.
You can buy in blocks of 500 (£4.25), 1000 (£8.50), 2000 (£17) and 5000 (£42.50) which is fair enough. But games on Marketplace cost between 400 to 800 points so it’s not like you can buy the exact amount of points you need. Grrrr.
From what I’ve read I think Nintendo does a similar wacky thing with their online service where as Sony actually let you buy things with cash!