I really don't think they've thought this through
Its BT my dear Milo. Do you think they think anything through, especially when it comes to dealing with their smaller customers at BT Retail level ?
Seriously, they've got their fingers in so many pies with their monopoly, and they know the regulator has no teeth ... so what does it matter to them if they loose a few BT Retail residential customers. Residential customers are just cannon fodder for BT, with the associated churn rate (e.g. they loose a few customers through the Yahoo dump, but they win a few through the new BT Sport .... the end result is either the same number, or likely more customers than they had before they dumped Yahoo).