The Offence
This charge is rarely used, largely because there are very few people who would cling on to a car if they're on toboggans, skis, rollerblades, etc., and even fewer people who would actually knowingly tow someone using their car. However, there are some incidents that have occurred in Ontario where people have been towed by cars. An example of what can happen if people are towed on toboggans happened in Burlington last year:
http://www.thestar.com/news/gta/article/648977
"Towing" is a strict liability offence. The wording is that no one shall "permit" anyone to be towed. If you honestly did not know that someone had grabbed a hold of your vehicle, that is a valid defence. If you also attempted to stop them, that is also a valid defence.
Knowingly towing someone, if they get injured, can also result in criminal charges.