Only reason i can think of the US sites won't ship it is that Iodine is a restricted product in most US states*, since its listed as a precursor chemical for meth labs.
Manufacturers of course dont have the same issues as retailers.
Although iodine IS a poison in large quantities, it's also a vital nutrient in the human diet. If you walk into any Millets or the like you will find iodine water purification tablets and drops so don't worry it isn't going to kill you or anything.
*only in concentrations above 2.2%. I presume since they're in crystalline form, this product is 100% iodine crystals or some iodine salt, and will probably fall well within the remit of this restriction.