I thought they were misdemeanor crimes [0] punishable by jail time.
So they are crimes, but not huge.
I’m not a lawyer or a law enforcement officer, but I thought that local law enforcement can certainly hold someone charged with a federal crime. Eg, if someone commits the federal crime of kidnapping, then local cops can detain that person until transferred to FBI.
[0] https://www.law.cornell.edu/uscode/text/8/1325