Technically they are responsible for policing provincial roads, enforced by the OPP.
A bigger contributor to this tint problem is the lack of a clear determination of what is legal/illegal. In Quebec, the limit is 70% allowable transmission of light, in Ontario it's only legal mumbo jumbo dealing with obstructed views, apparently leaving it up to LEOs discretion.
Department of transportation? No they are not. Police polices the streets. Ministry of Trasnportation passes law/regulation, renews licences etc.
Lack of clear determination what is legal or illegal?
Please read Highway Traffic Act before making assumptions
Section:
73(3) of Ontario Highway Traffic Act. The VLT rate or Visible Light Transmission rate of the vehicle's front and side windows should be 70% or higher. This means that the darkness of the tint cannot be more than 30%
Basicly in Ontario windshield and side windows directly to left and right of driver cannot have any tint applied legally. By nature an UNtinted glass already from factory reduces 15-25% light transmission with a meter.
Any tint applied including "legal" 30% is illegal as it will reduce way more light than 30%. Most readings of 30% tint would read 70% light reduced with a meter.
Back seats side glass and rear (hatch) glass can be any tint reading legally.
Also side view windows to driver cannot be obstructed. This is to be easily seen by LE in a traffic stop and possibly other drivers while driving as to see intent and state.