How and why? There are other apps that do that (Netflix is a great example) and this is just Apple being greedy.
Their distinction between the two is hilarious at best. If Netflix wasn't huge and was just starting they would do the same to them. Apple is wrong here, its just pure greed.
But we know thats whats Apple is all about these days.
The difference is quite simple. If Netflix and Spotify pull out their apps and are only available in competing platforms, then the huge installed base of those platforms might consider migrating to other options where they can use their platforms. If this email reader does so, then no one will flock to Android to get it. It is just Apple bullying the little guy that has no other recourse, while bending to the big guys that could actually hurt their business. Is not about being "reader" app.