The guy is an idiot, and obviously doesn't understand how Usenet works. Tell him to check out
Google access to the NG's. They run essentially similar scripts to this. So do some other NG services, but I can't remember them offhand. The biggest one was Deja.com which was bought out by Google. There is essentially no difference in what they do and your script.
In fact there is no real difference between what your script does and a persons individual newsreader, whether that be Agent, Xnews, or whatever. Some people even use Outlook Express, (and wonder why the rest of the group get testy with them when they ask lameass questions - especially in the binary groups where OE isn't suitable). All these clients allow a person to read messages and download files on these servers, and it doesn't matter whether a person uses a client on their own machine, Googles site, or your site.
You might want to look up your newsgroups on Googles' interface, and give him the URLs. (Dunno whether they carry binaries though as I figure that would be too much of a bandwidth load).
Let him threaten Google too and see how far he gets.
Newsgroups are public domain. If he wants his posts to remain only in the 'group' he is posting to then he should join a private mailing list or msn or yahoo group. Usenet works entirely differently and he's just making himself look like an ignorant arsehole.
Tell him to sod off, and don't go worrying about it.
Phoenix