When you dont pay anything , usually you end up as the product, meaning someone could be using you.
Take Youtube for example, it costs nothing to upload a video or to view a video , but inadvertently it creates a self contained system where lots of viewers (like you) who view their digital content (as mentioned in T&C they now own your content ) means lots of ad revenue for Youtube and its parent company.
Same applies for Search Results in Google, where the service (search) is free, but the fact that you click on links on the results page means dolllar revenue for the parent company.
To the average individual this is the way the world works, nothing unethical about that, but what such an ubiquitous (i.e widely used) global practice and shopification of the knowledge ecosystem known as the internet means is that ultimately human beings are reduced to a commodity, and some (those with access to digital infrastructure, services and financial clout) have more value (hence more useful and likely to be commoditized ) than those who are not so dependent on the internet (as an example, the poorer sections of society , the agrarian folks, certain sections of the industrial sector etc)
Bottomline - Nobody does anything free for anyone, there's always a commercial logic behind it, so pay attention , you might actually be part of a target audience and you might be like a pilot implementation (most of us in it services adopt this approach to learn the ropes , again in my opinion its an ethical practice as long as you are aware of what the Terms and conditions of engagement are)
If you need an app-review do DM.