Why on earth would it be piracy? It's an unofficial port, sure, but how can you pirate a FREE PC port of an XBLA game that hitherto never existed on PC? How do you pirate a freeware game, anyway?
If they were charging for it, on the other hand, there might be some problems, but since it's free, well...
The PC version is a different thing from piracy. It's barely even copyright infringement, especially since it seems that nobody holds the IP rights to the original game anymore. Anyway, since it's free, I really wouldn't worry too much about it, myself. Download and rage/enjoy!