The principle of the Amazon One-Click purchase, as outlines in Krugs "Don't make me think" is important here, I think. OK, that process is different, because it requires an accout to have been made, but the priciple is that putting no obstacles in the way of buying stuff provides a substantial portion of their revenue.
If I have made a decision to buy something, the less I have to do to make it happen, the less chance I have to change me mind. Consider each time that the user has to consider something other than "buy this now", they may get bored and go away. When they have to enter an address, they may get distracted. When they have to get their credit card, they may get distracted. But more importantly, when they have to register - a process cognatively discoected from buying - they may get distracted, expecially if the registration is more than is needed for buying.
It is fairly common to allow a no-register purchase, and then let the user add a password at the end if they wish to persist their account. That focusses their attention on buying, while gathering all of the needed information for an account as a side-issue.