I guess i'll just have to ask someone else...
I have the first edition, they have rearanged the paragraphs in the second edition, it starts page 149 and page 157
"The rule that IP has on MA media is that a router should always advertise the actual originator
of the route in case the route's source is on the same MA medium as the router that has just
learned the route."
There is no rule on IBGP , I think you can figure out why.
The behavior is only when you peer on the physical interface because then BGP can see that it is multi-access. When you peer via loopback, BGP cannot make that assumption anymore because technically as long as you have TCP connectivity to the other peer, it does not matter how you get there, since in theory you already worked around the TTL issue with multihop or opening up the TTL security parameters.
Now, with that out of the way, the way that iBGP and eBGP make advertisement decisions are fundamentally different - and this is for good reason. The entire concept behind the matrix of iBGP route advertisement decisions is wrapped around loop prevention. iBGP is not meant to be a dynamic protocol inasmuch that it should rather be a deterministic protocol - you should know how it is going to behave in just about every scenario before it occurs.
If you think about the context in which BGP makes it's decisions, you should find some clarity in why iBGP and eBGP behave different here. They both rely on underlying connectivity, but typically iBGP has an underlying IGP to operate on top of - which leads to a lot more connectivity knowledge. Setting third party next hop is much more feasible when it is reasonable to presume that the other neighbor has similar information. eBGP typically does not have that luxury - as typically it is a point to point connection. Thus, the only time it makes the assumption to change the next hop is when it has firm knowledge that other hosts can reach that next hop - and without knowing what their routing table says - they can safely make the assumption the connected route will work on the other neighbors.
This is just my stab at a guess, I was not in the room when they designed this protocol.