First it was a setting in the preference panel, preventing you from installing non-MAS apps without disabling it.
Next it's the upcoming rootless OS X, System Integrity Protection: it's only a matter of time until the ability to install non-MAS apps is completely removed, buried, or hidden in Recovery mode (as the SIP setting is)
I suspect this will happen within the next one or two major versions of OS X.
And we have heard this for how long already? Doing so would basically be suicide for the Mac. First of all because a sizeable chunk of users are technical users. Secondly, a lot of software is not available in the Mac App store and likely will never be (I think Microsoft and Adobe would rather abandon OS X than giving 30% for each cloud subscription to Apple and being at the mercy of the MAS gatekeepers).
http://widgetsandshit.com/teddziuba/2010/05/the-future-of-ap...
It's not "yet" -- it's now, with each release, and getting worse each time.
Pretty sure the default is MAS + Signed Apps. This setting also doesn't actually stop you from installing a non-signed app. You just have to right click and select open to bypass the warning.
I actually keep this enabled so I know if an app isn't signed. So installing non-signed apps is a conscious decision.
I think that you also need an admin password, which can be an issue for users who don't control their machines. (My work distributes Macs with users configured to be admins, but on Windows machines only allows standard users, so I assume that it's only a matter of time until they change policies and this bites me.)
Until then I find them to be the best laptops on the market.
The rest is pure speculation.
The SIP setting is most certainly "opt-out." Have you demoed El Capitan?
The Preferences -> Security -> Allow non-MAS/non-signed apps setting is most certainly "opt-out" on any machine you buy from Apple or at the store.
Prefacing something with "I suspect" generally allows one to speculate.
The trend is that you're gradually losing control of your machine.
