My theory about why the newer versions allow you to do this:
I think it assumes since you bought a calculator, and the other person bought a calculator, you then both bought a license to every OS update, and you two trading them would be legal under that license. At least in the standpoint of data transfer. So I guess they just wrote in support for this kind of transfer in the OS at a certain point, when they realized that not everyone can get to a computer to update the OS, and a teacher may be able to help them out or something.
Theory about the much older versions:
They simply didn't think of supporting this