Oh, that's why.

UPDATE: Commenters, as usual, have gotten bogged down in a general Israelis versus Palestinians debate. What I meant to point out is that the terrorist groups try to exploit any vulnerability on the Israeli side, regardless of what that means for the quality of life of the Palestinians they claim to represent. The elderly, children, and women, used to get more or less a free pass at Israeli checkpoints, because the terrorists used only young men. But once the terrorists figured out that young men would be checked thoroughly, they started using children, women, and now even elderly women. Which means that Israel will have no choice, security-wise, but to now check these groups thoroughly, too, making their lives more unpleasant (and opening them up to the abuse that unfortunately occurs whenever you give 19 year olds with guns discretionary authority). Similarly, whenever Israel tries to open up the Gaza border crossing, and he Erez industrial zone that used to provide thousands of jobs for Palestinians, the terrorists attack there almost immediately. The saddest thing is that "the terrorists" are not some fringe group, but the Palestinian government, as well as the opposition factions. And of course, they then turn this, actions they intentionally provoked, into a propaganda victory: "look at Israel 'harass' old women at checkpoints! look at Israel refusing employment to Palestinians!"