TLDR; The light stays on because Mazda. It is only a label for the other lights adjacent to it that are usually off.
I can understand the confusion about this light, but here's a longer explanation:
First, I don't know why Mazda decided this should be on all the time.
If you watch the overhead console when you start the car, you will see the 3 rear seat seatbelt warning lights, along with the white text 'Rear Seatbelts' (I think the words are right - I'm not in the car now). Next to that is, also in white text 'Passenger Airbag', which is what we are all angsty about. To the left of it, REGARDLESS OF WHO OR WHAT IS IN THE PASSENGER SEAT, is an icon in, from memory, orange, with the word 'ON'. This indicates the passenger side SRS system is active. If you have deactivated the passenger SRS system by using the auxiliary key from your key fob to turn the switch inside the glovebox, then instead of the 'ON' icon, you'll see a different, not sure if its red or orange, 'OFF' icon. A short time after starting the car, the seatbelt text and icons, along with the airbag icon (ON or OFF), goes out, leaving only the white 'Passenger Airbag' text to annoy us. That text, as annoying as it is, is simply a label, not a warning, and strangely, Mazda chose to leave that one lit, while extinguishing the seatbelt one next to it.
Something to consider along with this, is that while there are clearly exceptions across the automotive world, and perhaps even within the Mazda ecosystem, there are some generally standardised conventions that can be applied to our cars and what they display to us. Generally, anything lit or written in WHITE is for information, or a label. Think stuff like the PRND lights, speed or tach and other numbers on gauges. Stuff lit in green, is something to note, like the indicator arrows flashing green, or the green outline around the i-ActiveSense icon (which funnily enough, is white). Then there are orange icons or lights, that highlight cautionary information, low fuel lights are usually orange. Then red ones, that highlight a warning, or potentially serious information, low oil, seatbelt not fastened etc. All of this follows with the written convention in operation or maintenance manuals of 'Warnings', 'Cautions', and 'Notes'.
Cheers all.