Apple’s App Store has very tight security and a strict review process. It’s not infallible but major issues are rare.
And with that, keep in mind that the developers charging upfront for their software just want to make good software in the same way that a restaurant wants to make good food for you to eat. If they can make money there’s no need for less reputable ways of making money.
It’s the apps that are free and have no obvious business model that are worth considering. If they’re not charging for features or displaying ads, then you might wonder how they’re making money.
And in many cases they might not. A social network will gladly lose huge amounts of money for the chance that they might make it back further down the road.
Thats not really intended to be warning or anything, just that supporting the developers trying to make an honest living is a real benefit to the ecosystem. And they appreciate it too!