All cargo on a vehicle needs to be secured. In Canada, National Safety Code Standard 10 is the guideline for cargo securement. Most provinces, when writing their load securement legislation, just adopt Standard 10 for load securement rules.
Flatbed truck drivers understand the importance of load securement. There is nothing other than the securement devices keeping the cargo on the vehicle, and the securement is easily seen by the public and law enforcement officers.
Other commercial trucks must still secure their cargo to the same standards as flatbed operators or the rules for the specific cargo (if applicable). Just because a driver is pulling a van, reefer, or other enclosed trailer does not mean they do not have to follow the cargo securement rules.