Why do I have to sign into Ubunto One in order to install Steam?

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http://askubuntu.com – The first time I installed Steam for Ubuntu was during the beta phase, and I can't remember I had to do anything special. A few months later, I'm now on a fresh install of Ubuntu (13.04) and want to install it again. First ascertainment: it seems that APT doesn't know about Steam: Well, ok, perhaps because it's not a free software there is some repository to add to APT before doing anything. Let's try with the Ubuntu Software Center, which sometimes can find more than APT (no idea why, by the way). For some reason, it will find Steam ! But while it's flagged as a free software, the only (HowTos)