There is no lockout device built into your tranny. If it is functioning properly you should be able to shift down into first at any rpm in its range w/o double cluthing.
There is no lockout device to prevent your tranny from shifting into first gear at any speed in its rpm range while your car is moving.
They do *not* all do this. I now have TWO subarus in my driveway that do not do this. BOTH of those subarus can downshift into first while moving, because they both have functioning first gear synchros.
And, yes, shifting into first while moving is generally not a good idea and does cause synchro ring tooth wear. If you have to shift into first while moving, double clutch.
Once again, there is no lockout device that prevents the car from downshifting into first. Read the manual. If you can find it (in the MY01 manual or the MY02 manual) then you can come to my house and kick me in the nuts. One interesting thing you might come across in the manual, though, is the claim that your new Subaru has a fully synchromeshed transmission...