So the 70% is combining "fully justified" and "partially justified," with the only other options being "not at all justified" or "not sure." There's no option for like "mostly unjustified" so lots of people who both sides the issue - or even favor Palestine! - are likely to say "partially justified," which is a very broad and vague statement.
Another headline you could write from this poll: "Only 27% of young people consider Israel's response justified - compared to 81% of seniors," or "50% of Americans think Israel's response was justified."
Polls are, as always, fake.