Yes, they knew what they were signing up for, but now most of them have decided to show they have the backbone to stand up and ask for a totally justified improvment to these paltry terms and conditions. And It would appear they have the support of most BA and other airline staff groups and their own passengers! The voices against this particular dispute seem to be very few and far between.
Given the huge disparity of pay between the new and old contracts, this group of staff were always going to one day get organised and cause BA a problem. This is a dispute entirley created by greedy BA management, and I would argue it will be all of BA that suffers from the negative impact and not only the CC, who sadly, actually have very little to lose.