I also strongly dislike nationalism, but anti-colonial nationalism in opposition to an occupying force is something a little different. I'm not quite sure where else a displaced refugee people is supposed to turn.
Yes, oppose Hamas, call out Islamism, support secularist Palestinian groups, advocate a secular state as the solution to the problem. But waving a flag that the British Home Secretary has threatened to outlaw (ludicrously, of course, but it shows that symbol's power) is very understandable.