RolandP SBB is accepting the ticket voluntarily, as a courtesy.
I don’t think it is just courtesy.
"Bei Verschulden des Transportunternehmens gemäss Anwendungsbereich, so zum Bei-
spiel bei Zugsausfall, Streckenunterbruch, etc. gilt grundsätzlich der Tarif 601, Ziffer 8.
Sparbillette (Flex) und Sparklassenwechsel gelten in solchen Fällen für die nächstmögli-
che Verbindung ungeachtet der Zugsgattung und allfälligem Umsteigen."
I while a go I actually emailed SBB to ask how this is to be interpreted and they basically told me that with “Transportunternehmn” they meant any transport company that operated in, and in to, Switzerland.
So taking the next train when your TGV is delayed is a right. However if you book in this case Paris - Luzern together you in addition indeed have the right to compensation if you are delayed at your final destination, and the railway has duty of care for the whole trajectory. So that is to be preferred.