I know why they do this

[alohacnc]

This is common in the machining world. (I'm a machinist)

You use a softer fastener because it's a heack of a lot easier replacing a fastener than re-threading a hole. Imagine if you over torqued your oil plug. It's not fun taking out an oil pan and weld on a new thread so you use a soft plug material and you can just throw it out if the fastener strips.