One is federally funded the other is state hence the distinction and a few different rules. Like I can drive cattle down a highway, but never and interstate.
Who sets traffic rules on the Interstate System?
Because the States own and operate the Interstates, the States establish the operating requirements, such as speed limits. The States also are responsible for enforcement.
The National Committee on Uniform Traffic Laws and Ordinances (NCUTLO), which includes State motor vehicle administrators and police officers, publishes a compendium of model traffic laws and rules of the road. The goal of NCUTLO is to promote uniformity across the States, but each State is free to adopt its own traffic laws and rules based on its unique circumstances.
There are more citations on that site making it clear the States establish the rules for Interstates.
The law cited earlier is on point.