> Should we fine them for failing to maintain their vehicle to your standards?
You might want to review existing laws. See, e.g.:
Georgia: http://law.justia.com/codes/georgia/2010/title-40/chapter-8/...
"O.C.G.A. 40-8-7 (2010)
40-8-7. Driving unsafe or improperly equipped vehicle; punishment for violations of chapter generally; vehicle inspection by law enforcement officer without warrant"
Ohio: http://codes.ohio.gov/orc/4513.02
"(A) No person shall drive or move, or cause or knowingly permit to be driven or moved, on any highway any vehicle or combination of vehicles which is in such unsafe condition as to endanger any person."
California: http://www.leginfo.ca.gov/cgi-bin/displaycode?section=veh&gr...
"24002. (a) It is unlawful to operate any vehicle or combination of
vehicles which is in an unsafe condition, or which is not safely
loaded, and which presents an immediate safety hazard."
This research appears to have happened in Missouri, where it's harder to find the actual laws on the subject. That said, I did find this: https://www.mshp.dps.missouri.gov/MSHPWeb/PatrolDivisions/MV... which tends to imply that there are laws to this effect that I cannot easily locate via internet searches.