Because in the past, developers have obfuscated their use of private APIs rather than removing them. They are threatening to ban people who abuse the platform. They aren't going to ban anyone making an honest effort to eliminate their use of private APIs.
Yep - instead, Windows is dragged down by 20-40 years of bug-for-bug API compatibility, even in the things that were never documented as public API and pretty clearly look like internal solutions that the author at Microsoft never designed for public use.
There is a good reason for how Apple approaches use of their private APIs, and for how Microsoft approaches use of their private APIs. Not saying either one is correct, there really is no answer, but consider that for any public API that exists, there may be undocumented details that some software somewhere relies upon whether they realise it or not. Apple is just taking a more aggressive approach than Microsoft here by detecting and disallowing private API use in their store so as to avoid issues down the line.
