Plusnet is nothing like EE BT dont sell PLUSNET products and is run as a separate retailer to BT.
Are you saying that openreach are the same company as BT retail.
Here is the BT Group annual report:
https://www.bt.com/bt-plc/assets/documents/investors...
On page 15:
Trusted relationships across our EE, BT
and Plusnet brands help us understand
customers’ current and future needs and
create new products to meet them.
Pages 226 to 230 give the details of "related undertakings", i.e. companies owned within the group.
On page 229:
British Telecommunications plc 100% ordinary
EE (Group) Limited 100% ordinary
On page 230:
Plusnet plc 100% ordinary
Openreach Limited 100% ordinary
Therefore, of course you're right that Openreach and BT Retail are not the same company; and that applies equally to Plusnet and EE. But they are all peer companies and brands under BT Group. It's a marketing decision for each retail unit whether or not they sell each other's products.