The only costs I can see that CN or the railway could defend on a producer car allocation is if that siding was not used for anything else--if no other industry player or grain company was using that siding. If a siding is a half a mile or a mile long and there's a grain terminal on it, then the grain terminal is using that siding.
There's no more cost for the allocation portion of that siding for a producer car loading site. They don't pay an extra tax on it. If the siding is not being used by any other industry player and there's a producer car allocation, in defence of CN the cost would be the tax of the siding and the maintenance of the switches.