Just because they may not have ICBM's today, doesn't mean they won't have them tomorrow. (Look at Pakistan, India - not quite ICBM-capable, yet, but doing their damnedest, not to mention the Chinese, who may have ICBM's at least capable of hitting our west coast already.)
One problem with the "cargo ship" approach is the ever-present risk of discovery, or maybe someone getting cold feet and ratting them out. An ICBM system would make that a moot point.