(b) Citizens
Except as otherwise provided by the President and subject to such limitations and exceptions as the President may authorize and prescribe, it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport.
Q: I'm not facile with US federal law, but there are often provisions in State law that provide if act x is deemed criminal by the code and a penalty not specified, then the penalty will be y. So even though there's no penalty in 8 USC 1185(b), there may be a penalty specified by another section of the USC.
A: The section in question only makes the act "unlawful," however, not criminal. I am unaware of a default civil penalty.
Q: I acknowledge the difference between saying something is "unlawful" and saying it's a "crime." I don't know federal law; in that arena, however, the two terms may be construed the same. At random (I searched for "federal crime"), I pulled 21 USC 841(a). It begins "(a) Unlawful acts. Except as authorized by this subchapter, it shall be unlawful for any person knowingly or intentionally..." So it's not clear that use of the magic word "crime" is required.
A: Note how 841(b) provides explicit penalties for violations of subsection (a). The provision of penalties makes those unlawful acts criminal. The 1978 repeal of the penalties related to 8 USC 1185(b) meant that violations were no longer criminal. I've yet to encounter an analysis that holds otherwise, including in reported opinions. The magic word "crime" is not required, but if it is not mentioned then mention of "felony," "misdemeanor," "sentence," or "Title 18" will do. See 8 USC 1325 and 1326 for further examples.
Q: The conclusion in your second sentence might be true, but I don't know enough about federal practice to agree that there is a federal law distinction between "unlawful" and "criminal." Maybe there is. Absent a more sophisticated understanding of how the USC is drafted, or a section-by-section examination of the USC to demonstrate there are no USC sections that prescribe punishments for "unlawful acts," or a citation discussing the distinction, however, I can't (yet) agree.