I can't add my old G3 iMac to my support profile.

When I tried to add my iMac G3 (early 1999) at supportprofile.apple.com, the page refused stating "We're sorry, but this product is inactive" (first screenshot).

At first I thought, this was simply due to the fact that its support status is obsolete.

Then however, I tried to enter the same serial number at selfsolve.apple.com, and it worked (second screenshot).

Also, I was able register my first generation iPod shuffle at supportprofile.apple.com, even though its support status is obsolete too.

I'd be happy to hear any suggestions what "inactive" means in this context.