The reason the law you mentioned provides no penalty is that it's ideas are part of those of the Western Hemisphere Travel Initiative and there are further laws that address passports that do establish punishments. https://www.cbp.gov/travel/us-citizens/western-hemisphere-travel-initiative
The Western Hemisphere Travel Initiative mandates that all United States citizens and residents leaving the country present a valid U.S. passport to U.S. customs officials upon their return trip. https://traveltips.usatoday.com/passport-requirements-leaving-us-21007.html
The goal is to track who is coming into the U.S. ,and the government can question you or detain you long enough to verify that you are citizen when you re-enter the U.S.
If the person trying to enter is actually a citizen, likely, no further action will be taken, but if there were no laws about it, the government would not have the right to screen people and weed out those who do mean harm. See the answer provided by Mr. Jairam that quotes the law you quoted here: https://www.avvo.com/legal-answers/do-i-need-the-us-passport-to-leave-the-country--1971846.html
Also, see this answer in the Travel section of Stack Exchange that also quotes the law you did.
What is the maximum prescribed penalty for violating these rules?
(Yes I know it's you. ;) )
After reading the comment that was not there when I began my original answer, I see you wanted a general overall view of whether a punishment can be levied in criminal law if the law does not specify a punishment.
The laws that do not offer a specific punishment are not criminal offenses. To be considered a criminal offense under federal law, the offense must be classified in one of the categories listed here: https://en.wikipedia.org/wiki/Classes_of_offenses_under_United_States_federal_lahttps://en.wikipedia.org/wiki/Classes_of_offenses_under_United_States_federal_law
In the federal courts you are charged by Indictment. That means that the case is presented to a grand jury and the grand jury decides if you can be charged. To present a case to the grand jury, the prosecution has to allege a violation of a specific criminal offense, so the prosecution is not allowed to present violations of other laws, like the one with no punishment you quoted. https://www.justice.gov/usao/justice-101/charging
Some states allow charging by Information, which means that the judge decides if there is enough information to charge you, but the judge is still only allowed to base his decision on whether you committed a specific offense.