Newer cars disable the passenger airbag if the weight of the occupant is less than a certain threshold (65 lbs according to https://owners.honda.com/vehicles/information/2018/Civic%20S...).
Some quotes from your source:
> Placing a forward-facing child seat in the front seat can be hazardous, even with advanced front airbags that automatically turn the passenger’s front airbag off.
> WARNING Placing a forward-facing child seat in the front seat can result in serious injury or death if the front airbag inflates.
> WARNING Placing a rear-facing child seat in the front seat can result in serious injury or death during a crash.
Not exactly. Lighter weight occupants are not supposed to be in the front seat wearing a seatbelt. The same places you're getting the warnings for not placing child seats in the front seat will also tell you not to put children in the front seat.
They try to plug it as a safety thing as a way to protect ignorant consumers from themselves.
My guess is that it's really just automakers CYA so they don't have to treat it as a safety critical system. There's really no reason to have that sensor then.