That may be part of the reason, but the biggest reason they're going with Steam for Linux has to be as a bargaining chip against Microsoft, if Microsoft ever tries to go with a Mac-style 'Gatekeeper' solution that requires you to buy applications only from the Windows Store. Gabe even said as much when they released it -- it was in reaction to Windows 8.
> if Microsoft ever tries to go with a Mac-style 'Gatekeeper' solution that requires you to buy applications only from the Windows Store.
This bit of misinformation has now apparently calcified into conventional wisdom; however, 'Gatekeeper' explicitly permits code-signing without publishing through the App Store, with no attendant restrictions, and signed software is accepted by the default security settings.
All other fears boil down to merely the possibility that Apple will change their mind and disallow 'sideloaded' software in the future.
> This bit of misinformation has now apparently calcified into conventional wisdom; however, 'Gatekeeper' explicitly permits code-signing without publishing through the App Store, with no attendant restrictions, and signed software is accepted by the default security settings.
