You are using the same logic as those who say the US can build any permanent military base in Iraq it wants.
This land was taken by military force. Most of it goes way inside the Green line, the internationally recognized border of Israel, and its borders before 1967. It is not belong to Israel. It would be as if the US built a wall that went into Mexico city, rather than along the border.
It destroys Palestinian communities. The wall is built on land that separates Palestinians from the land that they cultivate. It also separates Palestinians from each other. In some instances, it places many Palestinians on the same side as the Wall as most Israelis (how that could be explained as providing for Israeli security is beyond me).
The purpose of the wall is to make Palestinian lives so miserable they will leave. They will not leave. They will resist. See here
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=124x108698Finally, the US subsidizes Israel to the tune of $5 billion per year. This is very much a US policy issue.