I have no clue if the original package was to include it, or if the dealer was supposed to package one inside it and tick the tickbox.
i dont know who put the chip in the box, and ticked on the outside, but i know the chip was optional
A 4000 T users for example, would find the 3.1 ROM redundant
anyway.. an aweful lot of people already had 3.1(besides CD32 and A 4000 T owners i mean) therefore it was probably a logical thing to offer the upgrade to those people
AmigaOS 3.5 could have easily been made not to require Kickstart 3.1 chip.
firstly it would of ment more patches
it would also mean that there are two different previous versions to upgrade from floating around, the software would probably need to be written to patch different things depending on the ROM, which would make the software larger, probably slower, and more difficult to debug/maintain for the developers
Lots of ppl paid for 3.1 chip for nothing, in my opinion.
(for those who didnt have 3.1 already) they bought it to use 3.5 ! :-P
--edit-- (changed 'obsolete' to 'redundant' to make it say more sense more)