Most people don't think about it because they never bump up against the invisible walls in normal use. Suddenly, they are on display for millions.
No grand conspiracy, just a widespread culture of "don't worry about how it works, we'll tell you if you win."
Apps with large install bases are implicitly valuable to the platform as they are popular with phone-buying customers. It's not in the platform's interest to alienate the developers of such apps as those apps, together, cause people to buy that platform's devices to run them. The incentives are aligned.
If I distributed VPN malware via enterprise certs, I would lose my developer account. When Facebook does it, they lose the enterprise cert.
Go look at the review scores of all of the top apps in the app store. None are below 4 stars, unless they are apps for services where users don't have much of a choice in apps, such as crap companion apps for hardware or national services that people are forced to use.