Unless the seat performs a structural roll, then I can not see why it is illegal to remove them.
Tony's comment about arse-holes is pertinent as they really do need a seat, especially around the road-bump hell that is here.
FWIW.both our Mazda vans had seats in the back, making them 5 seaters and the seat and sometimes the seat belts went in and out like a yo-yo depending on what we were doing with it.
Also, the Nikki we won had the front passenger seat removed to make it easier to load and unload the shopping.
Plod never said anything about any of these. Caveat, the only time plod has ever pulled us over is recently because the camper trailer was too far out of rego, but it was their auto number plate checking that picked that up.