I can understand the 'lost revenue' scenario. But if that was the case, why not restrict the exit rows and bulk head seats? And have a look at AA - as SCH+ we actually get their MCE for free - which I think retails at more that a seat assignment for BA. Or maybe MCE is under threat now as well.
What I don't understand though is whether BA thinks all SCH+ holders only ever fly BA? They must know from their surveys that people fly other airlines too - LCC as well as full service competitors. There are alternatives out there.