What? Can you elaborate? I mean if you want non blocking IO from an fd in Linux, you can just do that. Not sure what defaults have to do with anything. Your code will still have to be written appropriately.
Default in linux is read()/write() and be blocked. Doing async is a lot more work and until relatively (to NT timelines) recently quite limited (select).
On NT the standard way is async, and doing things in the block-and-wait way is abnormal and unusual.
Defaults matter. Because that's what most people will do.
Linux use read(2)/write(2) for both blocking and non-blocking. Still don't understand what you mean by how NT does it. Either your code is written to accomodate Async IO or not.