Or is this a simple case of "Thats the way the author chose to do it"?
Pretty much. The author of a module can name it whatever they want to. Whether or not CPAN allows them to upload it under that name is a different issue (you can probably find the answer in The CPAN Faq).
As for why the URI module was named URI instead of URL, there is a slight difference between the two. More information on this is available here.