In isolation, it makes no sense. But considering the purchase decision as part of the scenario, it does make sense.

Allowing free standby without limits harms the airline's ability to price different flights differently, reducing profits. You want to buy the cheap flight and standby for the expensive flight, and the airline wants to sell you the expensive ticket.

If you were on Southwest, you would have to pay full fare (minus the purchase price of your ticket) to standby on an earlier flight, even just 30 minutes earlier. Count your blessings.

AFAIK all airlines will let you standby for the next flight if you miss your flight and show up at the airport within a reasonable time afterward. So if you want to play the "buy cheap, fly expensive" game, book the flight before rather than the flight after.