Of course, but then they don't get the exposure of the play store. And only people who explicitly allow "other markets" will be able to install from other sources. I for one have not yet dared to tick that checkbox on my phone.
Actually I think that option in Android should be more fine grained. There are certain markets I would allow, but I don't want to allow ALL markets. At the moment the choice is only "play only" or "all sources".
I wish it would be more like debian, for example, were I could add to a list of enabled sources.
It is not like checking that box allows other markets to just push stuff onto your device. It just gives you the ability to get things from other sources. You still have full control of not getting stuff from sources you are worried about.
I know, but I also know how well that worked on Windows (not at all). What if some apps figure out how to fake that prompt or somehow hide it. Or if I get to the prompt while I am tired or drunk and hit "OK" too fast because I was expecting something else. It is just one barrier less.
The top answer in this Stack Overflow discussion says that the core package installation APIs are locked down fairly tightly (such that third-party applications have to go through the prompting):
Actually I think that option in Android should be more fine grained. There are certain markets I would allow, but I don't want to allow ALL markets. At the moment the choice is only "play only" or "all sources".
I wish it would be more like debian, for example, were I could add to a list of enabled sources.