There is no legitimate food safety reason for a health department to investigate this.* From a sanitation standpoint this would be no different than smoking meat in house. If you do that with wood, no health department in the US cares where you got your wood from because burning it kills any pathogens that may be on or in the wood. Cannabis would be no different. It is obvious they are just doing this to fuck with the restaurant.
*I am licensed to teach food safety and sanitation.
*I am licensed to teach food safety and sanitation.