My understanding is that the airline would only get a financial sanction if the documents being presented at the border by the traveller should have been rejected by the airline doing their document checks.

The airline can only go so far with their checks, they're not expected to get loup glasses out and check if documents are fake or manipulated, and are only expected to to take the documents at face value married with what TIMATIC says. Immigration officers usually have the ultimate discretion and power to refuse entry. This could be intelligence based data that their immigration systems have, and entry can be refused irrespective of whether a traveller is asserting visa-free, visa-on-arrival, ETA, or visa based entry, or any other form of entry (possiblity with the exception of diplomats?)