Let's face it. How many of us have actually contributed enough to any FOSS projects to demand for free and premium services. I personally would rather donate to a FOSS app instead of paying Sync, but it's unfair to judge Sync for having ads and trackers when the option of removing them is there. There's nothing wrong with any dev wanting to make a decent living or even more than that, as long as they don't go overboard and are honest about it.
Many open source devs make money off their work. You can still sell FOSS; the F stands for libre, not gratis. There are packages for sale on Google Play that you can just download and compile off GitHub for free if you want to be bothered to do it.
I mean it's the whole reason we are here. That a non Foss service e.g. Reddit went to shit because the community had no control. But I mean in t this case it's very different. The overall platform is foss it's just that you can pay for a premium experience. Like you can buy a Porsche to drive around but it's perfectly fine to walk around