Example of SQL ITSELF being unclear:

mtassin showed that it's NOT usable within a Function, even passing a static seed value - (Non Deterministic here)

The Select Rand(),NewID() showing SAME values for rand, yet different values for NewID - Shows Deterministic behavior here, else would be reevaluated per line like the NewID() is

So yes, it might be that the SQL Function creation team didn't allow for a static seed rand value, but at very least being able to show that Microsoft didn't get it entirely consistant helps show how difficult the Deterministic/Non-Deterministic behavior is