I think it's an agreement between M'sia-Spore govt... U can't import cars directly from S'pore AND vice versa... But u can do it if the car has been 'legally' exported to a 3rd country before landing in Malaysia... S'porean registered cars sold here are not done legally, JPJ can anytime trace it down and repossess the car.
On another hand, u'll have no issues driving a Spore registered car in Malaysia.. U just have to continue paying the more expensive Spore roadtax and insurance. Insurance claim can be a bit of a hassle though.