The US entrance controls are obvious -- immigration and customs officers you must satisfy. Unlike some countries (AU), there are no obvious exit controls. But the US has always exerted jurisdiction over _exports_ so CBP officers can and do stop people leaving the US to inspect for prohibited exports. Just about any comp/phone might be seized because it contains crypto (5A002, 5A992 unless you have the resources to press the Mass Market exemption).
Even if CBP are being less @$$hole-ish (seriously, a job requirement), they can certainly claim electronics could be carrying data (mostly instructions) that are prohibited export.