If they ever get put in front of a judge for any kind of criminal behavior, it's another thing they can be slapped with if the judge wants to throw the book at someone (frankly its low hanging fruit in that regard). Up to 250K fine, 5 years jail time, plus a felony.
If required to register with Selective Service, failure to register is a felony punishable by a fine of up to $250,000 and/or 5 years imprisonment. Also, a person who knowingly counsels, aids, or abets another to fail to comply with the registration requirement is subject to the same penalties.