Thanks for reaching out!
As you can see in section 1.3.1 of Curie's datasheet (http://www.intel.com/content/www/us/en/support/boards-and-kits/intel-curie-modules/000022330.html):
"...The Intel® Curie™ module has a built-in battery charger, which is optional to use..."
So, it is up to the manufacturer of the board's to decide whether or not they implement the battery charging feature in their design. Curie's hardware does enable the designer to do so, and if you'd like to learn more about how to implement this feature, I'd suggest you to read the Design Guide for the Intel® Curie™ Module (http://www.intel.com/content/www/us/en/support/boards-and-kits/intel-curie-modules/000022331.html).
As for the question of why the battery charging feature was not applied on the different boards you mentioned above, I’d suggest you to contact the respective manufactures regarding this topic.
I hope this information helps you,
Yes, I'm fully aware of Curie capabilities and Design Guide.
Arduino 101/ Genuino 101 was designed by Intel (New Devices Group) so the question holds for you, Intel: why was it not implemented?
We have an update for this case.
The reason why the battery charging feature was not implemented on the Arduino 101 is because the standard Arduino I/O pin layout does not have the provision for a battery charger. External hardware is needed for multiplexing the charger interface to the standard I/O pin.
We would also like to clarify that the Curie charger is suitable for a certain type of battery, it is targeted for OEM usage. The developer may need their own charger/battery combo that suits their application.
I hope this information clarifies your doubts regarding the Arduino 101 design. If you have any other questions, please don't hesitate to ask.