Bingo. We supposedly have "security guards" and "security checks" around everything important, but they don't really do much (in the literal sense that they don't actually check much). Besides, is it that much better if someone detonates themselves at the gate to a street party or shopping mall with a crowd around than if they get inside? What if it's a particularly active street party whose attending crowd has exceeded the gates and there are sellers of food and souvenirs outside getting mobbed, too (like the one for Independence Day I attended last night)? Anyone can just walk in and blow everything up.
Any large, open concentration of humanity is a target. The Israeli solution to this is closed borders and a really fucking harsh military policy. Contrary to popular belief, America does have its own response: very low population density to reduce the value of any one target.