Are you refering to the 150F? it cannot be road registered however it can be rec registered in Victoria with additions of lighting, mirror and a honk honk clown horn! You would not want to ride a 150R on the road!
As for the 250X and 450X the reason for the sticker on the back stating that the bike is not suitable for road use (even though it can be fully road registered) is down to (a) Tyres (b) Engine maintenance. These are a HI Performance beast and the oil should be changed every 100km's so for a lot of people that could mean every day or second day!!! Also valve clearances probably 500-1000km's so you can see that it is not a road vehicle. The reason for the Full Reg is to allow you when compediting in enduro events to go down roads etc that may require full reg.
Have a look at a XR400M or a WR250R in the motard version if you want to ride on the roads.
Or the RX motovert, pull the pathetic 50cc motor and drop in a 160 YX or similar!