The reason for the distinction is that commercial transportation companies will have to send the individual who cannot enter canada at their own cost.Eugene1985 said:If that's the case, so why is there such a difference between commercial vs non-commercial transportation docs requirements (non-expired PR card or PRTD vs.
any proof of identity and status, respectively) ?
Let's say you take a flight to Canada, have no visa, no eTA and no PR, the airline will have to send you back to where you come from at their cost. In order to prevent that, Canada put in place rules that the airlines follow in order to ensure that they do not have to face such costs. Therefore the rules to board planes and commercial transportations that cross the border are fairly straightforward and the documents accepted for travel are on a shorter list than what IRCC will accept to check your status.
For the rest of your post, you are wrong, no matter how you want to present it, there was no need/possibility for PRs to apply online for anything remotely close to eTAs.
For the post you refer to, you have been asked earlier to disclose your citizenship for that very reason. You refer to the exception to the rule. At this point, citizens of the US, a few french citizens and a couple of other exceptions exist to the eTA rule. If you are a US citizen, then this whole discussion would have been much shorter if you answered that question earlier.
If you are not a US citizen, then there is no point in arguing about the "technical possibility" of PRs to obtain documents similar to an eTA as the only thing that existed was and is the PRTD.