That 100 Wh is the guaranteed limit based on international norms. The same IATA norms allow batteries up to 160 Wh with the approval of the airline. Some airlines have enabled that as a matter of policy, some require a special permission for larger batteries, and some don't allow them at all (except as dangerous cargo). The 160 Wh limit is also quite new, and some countries may not have implemented it yet.
It is also part of the CFR; and it makes sense the IATA and CFR are compatible. My point was that a 70WH the Ais could have a larger battery but that was a design trade off.
If you travel with a laptop with a battery larger than 100 Wh, there is a risk that you have to abandon the laptop before you are allowed to board the plane. Especially when something unexpected happens, such as when your flight is rerouted. As a result, you rarely see large batteries in the default configurations of mass market laptops. They tend to be limited to specialist equipment for situations where you have to make special arrangements before flying anyway.
Correct, as the CFR states up to 160 but over 100 is at the airline’s discretion. Unless airlines decide to allow larger batteries no manufacturer will make them as a non-removable and replacable component for teh reason you suggest. It’s easier to carry 2 100WH batteries and not have to worry.