The isp doesnt need sell that product to be able to order it from openreach.
What ronski is saying that if the isp has a reason to believe the line will not exceed a 40/10 sync rate then to save themselves money they order the cheaper product from openreach.
I do agree with you tho they on shaky ground doing this as this counts as an artificial cap and means they not actually supplying a up to 80/20 service to the customer.
Both BT business and plusnet are BT group owned entities. So it is no surprise they follow the same practices. I got no idea if the likes of talktalk and sky do the same thing.