> The platform is open source. So if you want to use someone else's tools then you shouldn't have to pay this part, right?
The platform here includes "android", so yes if you aren't using Android you probably don't need to pay to support Android development. It's not clear here, but do you think "open source" somehow means "doesn't require funding to develop"?
> The cost of providing this is negligible. Many alternative stores and repositories do it for free.
Few/none of those provide app review and various anti-malware stuff. You can argue that these have negative value, but they absolutely have non-negligible costs.
Also I don't think any alternative stores support canarying/progressive rollouts of new versions, which is useful for developers and a nontrivial to support for other stores.
