Interesting concept that you see just about everywhere.  I think it began with supermarkets.  Just about every supermarket in the country rewards people for buying less by providing "express checkouts" for let's say, 10 or less items.  Ever go up to one of those express lanes, and put something back so you could make the under 10 quota?  On the other hand if you buy dozens of items you are left waiting forever in line.  On the internet it is the same, I often see white papers, pdf files and so on available with just a click.  And then comes the line.  Usually a form asking for your name, email, address, website, first born child, and favorite beer.  These are usually mandatory fields.  I don't fill them out.

Likewise when it comes to check out, some sites require a rather elaborate registration just to purchase something from the site.  This is usually after you have your cart filled. When this is a mandatory step, I often skip the purchase all together, unless of course, I am out of milk.