Passenger handling fees? Sure, the $75 almost certainly would cover any incremental AIFs, and other per capita fees assessed by the connecting airports, but its not literally just $75 in AC's pocket.

But seriously, they just pull the connecting flight coupons if moved to the non-stop, but if moving from a non-stop to a connecting service, they have to generate new coupons. Lots of staff don't know how to deal with this properly, so they use some sort of authority/override in their system to spin up what amounts to an unrestricted Y ticket -- the 'payment' for which gets reconciled in the back office. The passenger due to the Y ticket gets an un-paid-for LT07 entitlement or even an op-upgrade if they have status. Etc. Its easier just to discourage the practice or for staff to not do it, than to do it properly. Not that many CSR staff actually know how to do many tasks relating to ticketing because there's almost no training investment made by AC and on-the-job learning opportunities are usually clouded in extreme chaos like IRROPs. The disciplinary culture comes down heavily on mistakes made that might cause passengers to have their entitlements elevated beyond what they paid for -- something that most AC staff don't want to deal with the hassle of either. And last but not least, maybe the passenger is doing some sort of hidden city scam if they SDC a non-stop to a connecting, or otherwise deriving additional value from the connection that they haven't paid for (ie: having a brief business meeting at the intermediate city!).