One would think DOT inspectors know of this, if your driver gets pulled for inspection and has canadian plates but is hauling an interstate - they would audit the paperwork much more thoroughly (?) Considering this situation you described and the office edits the documents - this just lets the driver have a better case. They told me to do it and said its all legal, even sent me a BOL to provide to the customer. It really puts the company on the hook for all the fines and legal consequences.
I think this is the best answers we got so far by 
@LrgCar and 
@Freight Broker (thanks).
Its all up to the discretion of the DOT officer, I dont see border patrol ( CBP or CBSA) really caring enought about this. But if the DOT pulls you over and discovers you are moving interstate - it will all depend on how upfront and wise you are with the story. Nevertheless we dont recommend doing this, especially for companies that promote it as "legal" to the drivers! Think of the worst case scenario, this driver could lose the ability to travel into the US, potentially resulting in loss of future jobs. I am more than 100% sure that crooks who do this, will also tell the driver they dont have anymore work for him.
Be mindful of others and they will do the same to you!