You are absolutely right. I've raised this point with Enta before and it was one of the reasons Enta chose not to introduce a cease charge when BT brought them in first several years ago - the decision was made to keep the activation fee at £40+VAT rather than reducing it in line with a BTW reduction on the understanding that this would offset the costs to Enta of absorbing the cease charges.
As most "ceases" are migrations away which, except for migration to LLU, cost Enta nothing this worked well.
I've still not seen details of the actual BT price increase so I don't know exactly what the costs are now.
Perhaps the proportion of ceases that are migration to LLU or complete ceases has risen significantly. I just don't know however as Enta have not consulted any resellers on this.