In practical terms, you're probably right. I also haven't heard of a situation where the airline came after a passenger for a fare difference because, for example, a minimum stay requirement wasn't met. I'd also think it's fairly rare for the airline to try to collect a fare at all from a passenger that was denied entry but traveling on a one-way ticket. In this case, they'd certainly be returned in economy, regardless of the original class of service. However, if the passenger has a round-trip ticket, you can take it to the bank that the airline will consider the flight back to have used the return portion of the original ticket.