There's a very simple reason you can't stand by for a later flight.

Standby does not guarantee you a seat. If you stand by for an earlier flight, and you can't make that flight (because it's full), you can still make your earlier flight.

What would happen if you were to stand by for a later flight (and therefore don't show up for your original flight), and then every later flight is full?

Pay the $75, or fly your originally scheduled time.

If you were to miss your flight, three things are possible:
1. There's space on a later flight, and you're confirmed on it (possibly with the SDC fee)
2. There's no space on a later flight, so you get to play the standby games until there is space (noting that as E75K, you're pretty high on the standby list) (possibly with a SDC fee)
3. You have to buy a new ticket