I don't think it has anything to do with what's natural. At least not anymore. Whatever the original reason, it's self-perpetuating now. Most websites have the login form on the right-hand side of the page. Therefore, if you're striving for the goal of "don't make me think," you should put the login form on the right-hand side of the page...
...thus increasing the number of sites that have the login form on the right-hand side of the page and the strength of the suggestion to put the login form on the right-hand side of the page so your users won't have to think...
...thus increasing... you get the idea.