well iPhone 4 backs are extremely easy to take off, unlike previous iphones where you basically need to dissassemble the entire phone starting with the screen, and void warranty to get to the back, the iPhone 4 back comes off by just unscrewing the 2 bottom screws. also Its possible to replace the back cover without voiding warranty, or leaving any evidence it was ever replaced.
If iphone 4s had the capacity laser etched on the back, apple employees would just look at the back of the iPhone to see capacity to know what capacity iPhone they should replace it with (i have seen them do this). Even if they go into settings to check something else, there is still a chance they will not look or forget to look at the capacity in the about section.
If the iPhone 4 had the capacity etched on the back cover, a person could have just replaced his 16GB iphone's back cover with a 32GB one bought off ebay or something, ask for a replacement for whatever reason, and there could have been a good chance the "genius" would just look at the back of the phone, see that it is a 32GB unit, and give a 32GB iPhone as a replacement.
... or they could have decided not to etch the capacity on the back for cosmetic reasons
If iphone 4s had the capacity laser etched on the back, apple employees would just look at the back of the iPhone to see capacity to know what capacity iPhone they should replace it with (i have seen them do this). Even if they go into settings to check something else, there is still a chance they will not look or forget to look at the capacity in the about section.
If the iPhone 4 had the capacity etched on the back cover, a person could have just replaced his 16GB iphone's back cover with a 32GB one bought off ebay or something, ask for a replacement for whatever reason, and there could have been a good chance the "genius" would just look at the back of the phone, see that it is a 32GB unit, and give a 32GB iPhone as a replacement.
... or they could have decided not to etch the capacity on the back for cosmetic reasons