Can they though? Android does not make it difficult at all to install apps through stores other than their own Play Store. A quick internet search reveals that there are at least a dozen other options for users to download apps to Android phones.
I agree that a devices OS should allow alternative means to install apps, but Android has clearly done this.
The problem is, if the developer wants its software to reach the wide audience of the platform, its a obvious choice for him to use what the users perceive as the "official way" of installing software. Sure, you can offer both ways, but if your app is missing from the official store, the majority of users won't even know it exists.
IMHO we, as users and developers, have our share of the blame for problems like this one, and we should be vigilant so Android doesn't become like iOS.