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<@U9360TNKY> Because you cannot `apply` map. `map` creates a new observable but you cannot return it from `apply` (apply returns receiver instead), so actually you just call map and ignore result. 
Also, you cannot map behavior subject because this operation creates a new Observable, so you cannot push values to it, this is not a subject anymore
So the only way to create a new observable each time, like in your first example
As alternative solution, why not just add already filtered values to behaviour subject, subject is private, so you can control that