Because they buy at a discount and put them on the bill at retail. Standard practice.

Also, they need a blanket policy. They don't always know the source of your new parts but they know where theirs are made, so they know the warranty and other details.
One cannot take in some customer parts and not take others.

Their shop, their rules.

My non Porsche mechanic is pretty darn good so I think he can handle swapping out the struts and springs. Heck, I did it on my 00 Box S and I am a low level DIY.

Now as far as torque goes, I will leave it to him to what he wants to do. Not sure why they wouldn't since I am paying by the hour. Maybe you do it long enough, you just know what feels right.