Can a suicide bomber do significantly more damage at an airport than on city streets? A hijacked airplane won't make it off the ground, and if the bomber is on the ground, what's the big deal? You can't prevent suicide bombings from happening everywhere, so it makes sense to focus the effort on where the damage would be particularly bad (e.g. 1 WTC). A bomb at the airport on the ground doesn't seem worse than Times Square.
Personally, I don't think that we should worry about lone suicide bombers unless it becomes an issue. Israel is justified in considering lone suicide bombers in their security planning, the US... not so much.
Also, it's a matter of practicality/trade-offs. A lot of airports are isolated, and it takes less money to secure an airport perimeter than it does to secure (using your example) Times Square. There is a middle ground. The attitude doesn't have to be "we can't protect everything from suicide bombers, therefore we should attempt to protect nothing."
