Because web apps are treated like second class citizens on mobile systems, but not on the basis of any merit, just platform holder control. Hence, blatant copies emerged on app stores.
This isn’t an honorable way to make a living.
Harm is not limited to business and finance. It's just as harmful when good, charitable works are subverted by companies looking for profit. If nothing else, consumers are harmed by having the superior free version hidden away, crowded out by the ad- and spyware-infested apps.
What those developers are doing is like charging an entrance fee to a public park, knowing many won't realize the park is free. But the park isn't a business, so there's no harm...