That's because your server automatically creates an alias of "www" to point to the "public_html" folder as well.
They do this because some people find "www" to be more intuitive to them than "public_html".
But they're one and the same.
They're "symlinks" of each other.
If you change something in one, that change is immediately visible in the other, because they're both looking at the same place.

So, best to just ignore, forget about, the "www" folder, and simply work with "public_html" instead.

Right now you've set your site up as follows:
- www.your_domain.com - gives the listing of folders, because there's no website files in that folder, because it points to public_html (this is also referred to as "the root of your site".
- www.your_domain.com/fro - gives Zen Cart, because that's where you put the Zen Cart files

As dbltoe said, if you wanted Zen Cart "in the root", you could move it using the tutorial he mentioned. Or just delete the fro folder and start your ZC install over again by uploading its files into the public_html folder directly (instead of into a subfolder).