instead of thinking about it like "the fedilab dev is charging me money because i use google play", think of it like "the fedilab dev spends time and effort on getting his app to appear on google play (including an app publishing fee), and would like compensation for that effort. the app is still available for free from github and f-droid."
i know of quite a few apps that charge money on google play, and are free from f-droid. this isn't unique to fedilab, and if i had an android app, i'd do something similar.