Since it was a free upgrade i don't see any software violations issues.
It still is. The download is tied to your account. You can of course, as you noted, install it as many times as you want since you can always retrieve it from your App Store account. But the license only allows you to install it on Macs you own. It's not all you can share.
(1) to download, install, use and run for personal, non-commercial use, one (1) copy of
the Apple Software directly on each Apple-branded computer running OS X Mountain
Lion, OS X Lion or OS X Snow Leopard (“Mac Computer”) that you own or control;
If someone were to install Mavericks on another Mac, and that person ever has to reinstall it, they won't be able to without the ID and password of an App Store account Mavericks was "purchased" under.