As long as users will balk at paying even 4.99 for an app, developers will make them free and then engage in dark business models like surveillance and terrible in-app purchases.
To be fair and stop “blaming the victims,” this is a classic hard problem in psychology: The purchase price is an immediate, objective cost. Surveillance and the possibility of getting sucked into in-app purchases are subjective, uncertain costs with consequences in the future.
In aggregate, humans are really bad at making those judgments, trebly so when other humans are rigging the game to make the judgment difficult to evaluate.