IRCC explains it in laymen terms right here:
https://ircc.canada.ca/english/helpcentre/answer.asp?qnum=1457&top=10
It's not really an exemption from the requirement to have a PR card as there is actually no such requirement. It's more that the risk of fines and costs of removal commercial airlines (who don't have direct access to immigration databases*) operate under doesn't apply to a person who knows that they have a right to enter Canada. If you own the airline or bus line, you can make them transport you, regardless of whether you have a PR card. If you manage to show up at the Canadian border, regardless of how you did it, as a permanent resident, you have an unconditional right to enter Canada pursuant to section 19(2) of the
Immigration and Refugee Protection Act (IRPA).
Also, US citizens can, in most cased, enter Canada with just a valid US passport even by commercial carrier because US citizens are eTA-exempt but the carrier still has a discretion to refuse transportation if they know this US citizen is a permanent resident.
* Technically all an airline or other carrier needs is your PR card number on the back side of the PR card. With this number, they can obtain an ok to board message from the CBSA IAPI system.