IIRC BA's inventory is available 360 days out. Meaning you can buy a ticket for June 1, 2018. So it isn't that seats are not available. W17/18 season will start 29 October in 2018.
Also I recall that it was the airlines that asked for the availability of seats clause, as they didn't want a situation where they would be asked to bump other passengers. It is also written to ensure passenger access if seats are available in that cabin, even if seats no longer available against that booking class.