1. If you have a contract with a city or other government agency to tow vehicles, you either tow vehicles when you are called or you are in breach of your contract and will probably never work for that government agency again.
2. If there is any truth to this story, it's probably a situation where there are so many vehicles to tow that Ottawa's towing contractor is overwhelmed by the workload and needs help. Any refusals are likely to come from OTHER towing companies -- especially if they've never had any interest in bidding on a city contract.
Government contracts for towing and road service suck anyhow. Trying to get them to pay you is a real pain in the rear and almost not worth the time and trouble. I one time had the state of Ca give me IOUs for two years in a row for service. And then they claimed the controller would only disburse half of what was owed and wanted me to agree to this.
I had to take them to court and they ended up paying interest on top of what they owed me. Never did another thing for them again. Everyone who got the contract after had the same problem. Sooner or later they run out of vendors to screw and had no one who wanted to do it for them.