Valid point. Even if Apple wants you to have the most up-to-date information for "security reasons", it would be more effective to perform continual syncing of that database to each computer.

If your computer's internet is offline then Gatekeeper could use that cache of information to make decisions.

The downside is that Apple wouldn't get any information about who opened which app at what time... which is arguably a privacy violation in itself.

I wonder if desire for that tracking information influenced the design.