You can either have it mapped in the robots.txt
e.g.
http://fanficslandia.com/robots.txt
Or you can submit almost any URL in Google Webmaster Tools
And .. if you have it enabled to ping Google, it would be autodiscovered anyway
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And about the why?
Having it in the root folder means you need to either chmod 666 the specific files (which is a mess for splitting), or that you have to chmod 777 the XenForo directory (which is a security problem), or that you have a proxy script and then you have to use mod_rewrite to reach it (which is additional configuration)
It's just much easier and secure this way
The only thing that it would be missing from the vb hack is the actual registry that Google downloaded the map. Since you can get that from Webmaster Tools anyway we are probably not loosing much