I'm not 'imagining' anything; there are forces of up to about half a tonne passed through the brake pad backings; since the friction material doesn't correspond with the piston area and furthermore since the pistons are very often not flat, there are substantial bending stresses in the brake pad backing. They need to be strong.
Bicycle disc brake pads don't usually have backings that are 'about 3mm thickness'; they are usually about 1.6 or 1.7mm in thickness.
A 'copper backing' (rather than an (expensive) copper alloy backing) might have about 1/4 the strength of steel, if it is cold worked. If annealed during processing (difficult to avoid if the pads are sintered type) it can drop to less than 1/10th the strength. At that strength level you would easily be able to bend the pad backing using your fingers.
If the pad backings deform in service, the linings fall off. You don't want that, remember.....
Possibly the backings you have are 'coppered' i.e. coated in a thin layer copper, to provide a little corrosion resistance, and this has been lost in translation.
A similar situation exists with wire brushes. Before now I have wasted a considerable amount of time trying to explain (to people who should have known better and who were trying to avoid any contact of steel with an sensitive item) that 'brass wire brush' and 'brassed wire brush' are not the same thing at all; the latter is made of steel with a brass-coloured finish.
cheers
