Hi i happen to work for BA customer support. I dont know the reason for the restriction but it has always been the case that you need so many eligible flights a year. I dont know the reason for the restriction but i think its fair to guess that if you join a frequent flyer scheme for BA that they expect you to take flights that will help their profits the most! If you check the link here at the bottom of the page it tells you number of eligible flights for each tier:
http://www.britishairways.com/travel...s/public/en_gb
This link then explains what is considered a eligible flight:
http://www.britishairways.com/travel...s/public/en_gb