Here is part of his explanation:
What do you think of this idea? Do you think it would work at its stated purpose (with restrictions on selling citizenship to terrorists, etc.)?
First consider the fact that America's homeless and panhandlers (who are often different people—some homeless don't panhandle and some panhandlers aren't homeless) are actually quite wealthy. Almost all own an asset—their United States citizenship—that is worth several hundred thousand dollars. The problem is that they are denied the right to sell that asset.
Citizenship in the United States is a highly valuable asset because it gives its owner enormous productive potential. American citizens are able to take advantage of the
opportunities to combine their ambition, ingenuity and labor with an unparalleled capital base and other hard-working and talented people to create wealth. The homeless and panhandlers in America have clearly failed to use their citizenships as productively as many non-U.S citizens could, and would, if they became citizens.