I think it is highly immoral that a potential customer should have to pay a company to compensate them for the strategic mistakes of one of their employees, no matter how long ago those mistakes occurred.
I wouldn't call it a 'strategic' mistake. It could be very much as Andrews says, that in the planning stage Openreach were going to fund the cost themselves and now they just need reminding of this.
You also have to look at it from Openreach's view point with regards to their excessive construction charges generally. Say they designate an area to be FTTP and believe the ducting runs to all properties in the area. Joe Bloggs orders FTTP and the Openreach engineers doing the external work find that a builder has poured concrete down the ducting, completely blocking the whole thing. Should Openreach be responsible for the cost of rectifying the ducting (which could easily run into the thousands if it's a large blockage and patios/paving etc need to be removed)?
This is one of the reasons why a widespread FTTP deployment is unlikely any time in the next decade. If every duct was clear (not blocked) and ran to every property, things would be so much simpler. Unfortunately, the FTTP trial areas have shown that blockage are extremely common and they are both time consuming and costly to fix.