I would imagine that it comes down to the logistics of managing all the inventory manually on a flight by flight basis.

Time to put IT geek hat on ...

There is probably a mechanism in the IT system for applying fare rules to specific fare buckets e.g. by applying a discount % when sales are loaded. In this case, they appear to be increasing the fare bucket to above 100%.

Zeroing the seats out probably freezes the inventory and prevents other processes from working e.g. online check-in, manage my booking etc... You would be surprised how complex and tied together processes are in enterprise systems that have been grown organically over time, especially when they interface to other enterprise systems.

When a process is overridden manually, it has to be put back manually. If BA does this for every flight over the course of 6 days or more, this will be a huge and costly operation in itself and will leave them very open to human error. I agree with BA on this one. Increasing the fares to a point where very few people will want to purchase them is far more sensible.

This is just my speculation / interpretation - I do not know the specifics of BA's IT systems.

...IT geek hat removed

Back to Simon Calder ... I think that he takes a very one sided view every time and, like the OP pointed out he does tend state the bleeding obvious.

BA are allowing / asking travellers who don't need to travel to change their flights to a later date free of charge. In my view this is a positive move. I don't think that BA are being "very naughty" or have any secondary agenda. They are a business and are in the middle of a very difficult logistical operation.