How do I prevent OS X from automatically installing app files on my drive I have just reinstalled my Air with Lion. I didn't want to do a Time Machine restore, as I really didn't want any of the old apps, and I'd like to manually sort through my home folder and discard old crud.

After copying back my previous hard drive contents to ~/PREV, I noticed that Spotlight would suggest to launch apps that existed there, even though I hadn't "installed" them, like copied them to the /Applications folder or run their installers (for those that have them).

I quickly updated the Spotlight preferences to not index the ~/PREV folder (no need to, anyway, as it was only a temporary folder). That fixed that.

Now, I just discovered that Finder has context menu items stemming from BetterZip, which is now in my Downloads folder, having been copied over from my PREV folder. I did not install BetterZip.

What the ....? Somehow OS X is automagically installing .app files whereever it finds them? How on earth is this a good idea? What is OS X doing behind my back? And most importantly, how do I stop this crazy behavior?

Thanks :)
The Apps that the Finder shows in the context menu "Open with..." are not determined by Spotlight.
Independently of Spotlight the System scans for all .app files on your disk and builds a database of Apps and the file types they can open.
As long as you have an App on your disk, the Finder will offer to open files with it.

AFAIK, you can only prevent this behavior by either moving your apps to another disk, deleting them entirely or (what I often do) zipping them.

(By the way, OS X is not installing the Apps. Usually an App is 'installed' when it just is on your disk. Installing like on Windows is hardly ever necessary for an App to run on OS X)