It is an title ID, not an title key. Title IDs are used to identify applications, title keys are used to decrypt the applications. You could see the title id as another, unique name for the application that the 3DS understands.
Because the title key site also hosts encrypted AND decrypted keys that allow the dissemination of copyrighted content. You can't download an app/game with JUST the title ID and no 3rd party program. That's why you can share ID's and not the site, the ID isn't enough to get you the actual content.
System applications also don't need to be decrypted, as they carry legit RSA signatures and will install even on stock systems via userland homebrew. So there it doesn't even fall in the same scope as using the title key site as it only hosts free paid content. It's the equivalent of a warez site. Your question is like asking, "If I can tell someone about murder, why can't I commit it?"