Mizter T ) gurgled happily, sounding much like they were
saying :

If you check the floor, you'll see holes that allow water (or other
liquids) to drain out.

Now I presume people at night have a tendancy to throw up, or urinate
on the bus, hence the plastic seats and the drains. Or, perhaps it is
so the bus company can literally jet wash them in the depot. Or, both!

Said drainholes also exist on many other models of buses that don't
have any plastic seats.

Could, perhaps, be something to do with lots of people getting on with
soaking wet shoes and umbrellas which promptly start dripping
everywhere...?