I'm honestly confused by this line of reasoning.
I think I've mentioned this a couple times, but it seems identical to eating at a restaurant. If you voluntarily choose to work in Canada, then you sign up to pay CPP. Just like if you go to a restaurant and voluntarily order/eat some food. After you get the job or eat the food, you have to pay the bill that you agreed to.
Could you point out the involuntary part?
Since you asked...here's the way I see your analogy.
If the restaurant is the only way to get food in your country and the price always included paying for a ticket to Disneyland, I can see the parallel. In that case, would you consider buying the ticket to Disneyland as voluntary or involuntary in order to get the food you need to survive?
I don't consider that voluntary. It's required to buy a necessity (food), not an actual voluntary choice. And yes, theoretically, everyone could skip the restaurant and feed themselves by subsistence farming and hunting and thus avoid buying Disneyland tickets. That doesn't make it actually voluntary in my opinion. Even if Disneyland is providing a societal good by employing everyone who wants to work there.
So working in Canada (eating at the only restaurant) comes with the added cost of paying into CPP (buying a ticket to Disneyland). For most, working is a necessity in order to buy the essentials of life (food, housing, etc) and so they pay the added cost. But they pay that added cost because the alternative is worse, not because they actually want to. If it was voluntary, people would pay that cost even if it wasn't a legal requirement.
To try another analogy, if I'm mugged on a street and choose to surrender my wallet under threat of physical harm, did I voluntarily give the mugger my wallet?
I think you would say yes, that I voluntarily chose the hopefully safer route and that was my choice.
I would say no, it was not voluntary, that it was coerced under threat of physical harm. The coercion makes it involuntary even if I chose the lesser harm.
And, just in case anyone is interested, I am actually in support of the Social Security system in the US. But I don't consider it voluntary.
30 years ago, I was not in support of SS. But exposure to a wide range of people and their poor financial decisions for many, many different reasons, have convinced me that the safety net is worth the cost and loss of a slight amount of self-determination. I am not a Libertarian.