is this an internet secret code I am missing ?

It's even in the Urban Dictionary [1]:

> Linux ISO: A codeword for copyrighted material shared without permission, usually over P2P networks.

[1] http://linux-iso.urbanup.com/2782626

Many Linux ISOs are distributed via torrents.

Ask yourself what other types of media can be had on popular torrent sites.

The two things that torrents are best at are downloading Linux ISOs and pirating content. So using qbittorrent to download Linux ISOs is usually code for piracy, since torrenting Linux ISOs is legal and piracy is not :)

Piracy is legal if you're big enough

I think that's called being a privateer, you just need the right papers from the king!