In answer to your question as to "why" they request an email address instead of just forwarding you upon payment to a download page - it's so they can send you advertisements and sell your email address. It's designed that way on purpose. All that folks have said above is certainly true in a technical sense but there are other ways of providing you the ability download it. This is the reason they want the email address.
It's just a fact of life in the internet world of sales. Perhaps I'm cynical but I know I'm also right. Doesn't stop me from providing mine to them.