No, the US should not be allowed to request or demand access to a US Citizen's electronic devices for any reason whatsoever, no matter what Homeland Security says. The whole point of customs was to insure that goods were not brought into the country to avoid paying duties. The contents of an electronic device cannot be charged a duty. Anything else is beyond their authority. Of course none of this will stop a guy in a fancy uniform from demanding an illegal search anyway and making your life hell. Given the current government this will only get worse.