The apple and the Windows icon advertise these platforms. The more games have these signs the more people can imagine to switch systems to their preferences.

The SteamOS logo is simply the Steam logo. Everyone knows that he is using Steam and most people don't even take notice of it.
When I talk to other Steam users they believe I'm a fundamentalist Linux maniac playing 2 alphanumeric games in the terminal... When I tell them how many and which games I play without issues and which common programs also work they are regularly speechless and don't believe me because something that's free must have massive snags right? It would not happen if Tux was in place of the Steam logo or aside, or even in the logo in any way. It also shouldn't say SteamOS+Linux but SteamOS/Linux.

The Steam symbol is a good logo, however, it is not versatile and not appropriate to represent Linux. When people see it, they think of the Steam client, or Steam the digital distribution platform, not an Operating System. When it's listed beside OS logos, people mistakenly think it refers to Steam Play or just ignore it.

I think Valve has done this intentionally, in the same way Google did not advertise ChromeOS or Android as Linux. Valve wanted to market SteamOS and SteamMachines to people who don't know about Linux.

In my opinion, this was a mistake and has only served to confuse people. Tux is already established as the mascot and logo representing Linux. It needs to be brought back.