I posted my theory on this one a couple of weeks ago.
The walls are permanent fixtures. Nothing passes them, they're always there. Therefor it'll be hardcoded that nothing can pass them. The same reason you bounce off the mess fences as hard as you do the concrete.
Here's my theory on the Pinball Effect (TM) (patent pending):
The red/white barriers are dynamic and can are moved depending on the track config or autocross layout.
The collision physics updates in cycles, but about 1/5 the speed of the tyres physics (IIRC). Because of server latency and ping speeds, the exact moment of the collision often falls between these updates and by the time the engine has realised you're touching it, your car has already effectively passed through it.
Of course it knows you can't occupy the same space, so it begins the collision calc. But because the car is touching all sides of the barrier (the front side you should've hit, the backside your nose is sticking out of and the top) it calcs them all. It's an impossible situation but it tries anyway, rebounding off one part, colliding with another, rebounding again - all in the space of a few milliseconds. Multiple forces speed the collisions up and eventually your ping/lag causes you to leave the object again and appear in the rebound direction of the last side you hit.
Might be totally wrong, but that's the way I understand it.