At the time when they had only just taken over Plusnet, it was about market share. PN products were offered bare bones, as that is all they were worried about.
Unless already knowledge, many don't know that EE and Plusnet are owned by BT. This is better for BT as it increases the chances that someone will see a "competing" offer and signup. There have been posts on here where people have gone from BT to Plusnet, not knowing they are controlled by the same co, although still different networks to a degree at the access layer at least but end result is BT retain the customer in some way.