To be honest it makes sense...
Look at it this way, if people choose to download apps outside the play store, they do so knowingly and also take on the inherit risk that the app may not be safe. For people who don't want to worry about malware and other risks, play store makes sense, its a place for the general consumer who wants to be able to download whatever they want, without incurring risks, a place where they know someone is controlling the content that is available and ensuring it is safe to use.
Apps purchased via the play store are screened for things like malware, similar to apple's (though much less stringent) approval process.
Its basically a security measure, if you want to sell your app via the play store and have it scrutinized by Google, then obviously all updates to that app should be subject to the same checks.
People complain about this, but the fact is that you can easily download apps from sources other than the play store without any modifications to your phone, something that can't be done on IOS without jailbreaking...
So yeah, Google wants to control the content they distribute, its completely understandable, if a developer wants to sell their app outside of Play, they have that ability, they're not being forced to sell it on Play... Facebook could just as easily provide a link to their app on their own site (Sirius radio does this for Canadians, its a direct link off the Sirius website, no play store involved).
Its not monopolistic behavior in any way, since other options are available.