An exception to what? I've read this article a few times, and I really don't understand the claims of this "commission." The fact is that the wall was built well within the boundaries of their property. Is the commission stating that their authority over property is, in effect, an easement well onto their own property?
What really pisses me off is the attitude of the US representative, in saying a) that he's got the job for life and can't be fired, and b) that they're not a government commission, and therefore not subject to laws or the Constitution.
IMHO, if an international entity is operating on US soil, it's subject to the laws of the US. Since these people are NOT diplomats, no diplomatic immunity is assumed.
Mark