It could be related to battery position, rather than purely electrical characteristics. The spaces for AA and AAA appear to be keyed for the cell diameter. Since everything is the same, electrically, except for the mAh, you can probably control the LED driver by using basic position sensing on the cell locations with minimal components and efficiency cost. An LTC3090, for example, could be used with relatively simple voltage dividers to adjust the ratio on the Vin and Vctl pins.

Batteries wired in series increase the voltage provided. The example in the OP us just a battery whose LEDs run at anything from 1.5V to 6V and accepts both AA and AAA batteries. It's not a foil to planned obsolescence, it's just smart design. It could still be made with the same design, but purposely use LEDs that die sooner, in which case it's smart design and planned obsolescence.