I feel its more of a bonus for seatbelts to be installed because
afterall,
1. PIW is rear facing, any sudden or hard braking will thus not
cause him/her to get thrown off the wheelchair,
2. The wheelchair has brakes, instead of the seatbelt, i feel
its more useful to paste a sticker to inform wheelchair users to
apply brakes while stationary such as in the uk,

3. The piw most probably will not use the seatbelt since its not
mandatory, something similar like how the seatbelts on the
rear-most center seat is not used by pax too.
Do correct me if anyone have seen piw almost getting thrown off
the wheelchair by not using the seatbelt.
I agree. It is more of a bonus but not necessary for such
facilities.
As long as the BC drives safely, there is no need to utilise a
seatbelt.