The airline can and most likely wll refuse boarding without a return flight - airline policies. This is prior to speaking with CBSA
What airline do you have this experience with, or can you point me to a specific policy? I guess the best place to ask the question then would be directly to the airline when buying the tickets on their policies.
I've travelled to Canada (and the US) as a non-citizen/non-pr many times (both as a visitor, and on a work-visa) on one-way tickets on many different airlines without ever being refused or even questioned by the airline. The important thing is to show that you have the means to purchase a return ticket once you're there or have proof of onward travel by other means such as by bus.
Some airlines may be difficult, but thankfully as consumers we have a choice and can always look at a different airline. Though, I guess to save any potential hassle with any airline the safest bet would be to buy a round-trip ticket. I still think it might even be cheaper to do it this way despite change fees. Two round-trip tickets might also even be cheaper than two one-way tickets if the flights aren't changeable.