Why are copper shower pans nailed to the framing?

I am in the painful process of re-tiling a shower that started leaking and after removing the tile we found that the pan had many nails through it to the framing. The shower was installed 20 years previously, so obviously these nails had all rusted up badly, although the pan itself was in good shape.

Now am going through the hassle of removing all the nails, which in some cases are nothing but hard mounds of rust and patching all the holes.

Why on God’s green earth would they nail the pan to the framing? I mean the thing had 50 pounds of cement and another 20 pounds of tile and the pan was jammed tight into the framing anyway. It’s not like it was going to be moving anywhere.