Saboteurs were a real risk, and not an entirely illusory one; in the event, however, the Axis powers achieved relatively little success in espionage or sabotage operations in the continental US. But hindsight is 20/20, and so a definitive conclusion as to whether internment or counter-espionage/civil defence measures were 'justified' is invariably a value judgement.

Popular fear of spies and saboteurs in the US were part of a complex web of public anxieties around infiltration of American society: the post-WW1 'Red Scare', and ominous reports in the press during the 1930s about the rise of fascism/Nazism and an existential threat to democracy only heightened fears of a fifth column insurgency lurking in the shadows.

But even more acutely, the American public remembered incidents of sabotage carried out during the last war: most notoriously, the 1916 explosion at a munitions depot on Black Tom Island in New York harbour, which killed seven people. Although suspicions of German involvement were widespread at the time, it was only in the late 1930s that investigators reached the definite conclusion that German agents were responsible — meaning that the memory was still fresh by the outbreak of the US' second war with Germany.

During the Second World War, both Germany and Japan repeatedly attempted (with varying degrees of success) to infiltrate the United States for either intelligence-gathering or sabotage purposes. The considerable success of the FBI's counterintelligence efforts — bolstered by J. Edgar Hoover's well-known love of publicity — meant that the general public were acutely aware of the danger of infiltration.

Almost immediately after the outbreak of war in December 1941, the FBI rounded up 33 members of the Duquesne spy ring run by the German Abwehr; in June 1942, an Abwehr sabotage plan, Operation Pastorius, was foiled and 6 out of 8 participants were eventually executed. On the West Coast, Japanese agents in Hawaii were involved in laying the groundwork for the Pearl Harbor raid — most notably Takeo Yoshikawa, whose role wasn't clearly understood until years after the war, and the German-born Bernard Kuehn, who was arrested by the FBI in February 1942.

Thereafter, however, the Japanese achieved relatively little effective penetration of the US; the Germans were in much the same position for most of the war. It's a strange sort of cognitive dissonance: the actual level of infiltration was relatively low, because US counterintelligence proved effective and neither Germany nor Japan were especially skilful at large-scale, long-range intelligence work — but the fact that their intelligence failures became widely known also made the public and the government more concerned about spies and saboteurs.

This fear was used by the federal government as the rationale for Executive Order 9066 (which authorised the internment of Japanese-Americans) and the enforcement of the unusually restrictive Alien Enemies Act and War Powers Act. There's also a complicated chicken/egg problem in there: it's difficult to say whether these measures, and a hyperactive civil defence programme — all of which were presented as reactions to a clear and present danger — contributed to the ineffectiveness of enemy sabotage/espionage, or if enemy spies and saboteurs were already ineffective and those measures were essentially overreactions.

In any case: the Japanese-American internment programme is (and has for decades) been widely regarded as a disaster, and a complete violation of Japanese-American civil liberties. But in another sense, you can understand the contemporary rationale; it just looks like foolishly blunt-force approach.

I do think you're on to something with your point about civil defence programmes making people feel active and involved in the war effort, but that's a civil-society/social history question that I don't feel especially well-equipped to answer. Superficially, though: I do think civil defence programmes were less significant than economic/industrial/social programmes — like the labour mobilisation efforts and war bonds — in mobilising and engaging the general public with the war effort. I think by design they were primarily intended to guard against the perceived risk of enemy action.