Roblox is free too, so you need to pay for the platform somewhere, which means it has to be part of the microtransactions. I expect to pay for Android and iOS while I buy the device, not during the subsequent multiple microtransactions, thus that platform fee is double dipping.
What one pays for Android/iOS gets them a device with calendar, email client, camera, web browser and not much else. Since one can extend that functionality later by installing more apps — including these that did not even exist at the time of buying the device — the double-dipping is double-edged. The buyer can get more functionality, and the platform creator can get more money.
I think when you buy an Android phone you aren't paying for the OS (based on a quick search it seems that Google doesn't get a cut of Samsung phone sales and doesn't charge OEMs any licensing fees).
Hence, why they're willing to offer the OS for free and why they push so hard for it. They make far more money from app purchases.
They might reasonably say they'd rather pay for the OS upfront instead though.