You are confusing in-App purchases with the actual price of the App. They are not related.
An App can be free, and still have in-app purchases, and an App can also have an upfront price and have no in-app purchases.
Then there are apps that have an upfront price and additional in-app purchases too.
An in-app purchase refers to any item, feature or service the app offers within itself as an additional purchase. It is not the actual cost of an App.
As mentioned, you can scroll down to the info section of the App in the App Store and see a list of in-app purchases the developers made public there.
Once you download the App, you can then check all the in-app purchases it has and see if you want to buy the additional item, service or feature or. not. In-App purchases are entirely optional. Nothing is forcing you to buy anything the app offers within it as an in-app purchase.
There's nothing deceitful there.