i've taken these flights, and i'm going to take a wild guess that the flights are probably 25% to 40% Bangkok "o&d". which would mean doing what you suggested would mean flights would be going long distances rather empty.
i guess BA figures "it's in our terms and conditions that we're not responsible for anything", so they can get away with inconveniencing a great number of people. and then, as usual, the protesters, who made their plans known many months ago, get blamed for causing all the trouble.

if BKK/thailand can't control their populace, and provide safe secure airports, the market will reflect this shortly, as tourists go elsewhere.

and these flight disruptions reminds me of a recent flight a friend of mine was booked on, on Tiger Airways, a budget airline i think based in Singapore, for a flight from Clark (Manila) to either Macau or Hong Kong. the friend booked it many months ago, very cheap. then they get an email the flight is cancelled, and they issued a refund. the friend scrambled to find an alternative flight, now at almost 3 times the price. doesn't seem fair to me, but i know many of you airline lovers think the airlines can do no wrong and shouldn't have to compensate in any way.

not too much more to say here, other than it's fun watching on CNN all the distressed passengers saying how awful the airlines are treating them, as the airlines just laugh it off