Probably on a theoretical basis yes, but in practice there's little accountability because there are enough "other factors" that prevent someone from complaining. For example, there can theoretically be...

• Pax with disabilities that get the reserved seats (e.g., on a 739 it is 10B and 10C that are blocked for pax with disabilities)
• Pax who have paid W tickets that did a same day change, had IRROPS, or did a SDC
• IRROPS for pax in F that agreed to downgrade to Y/W to get home sooner
• Other things I am probably forgetting

Since customers can't see who on the "standby" list meets the above criteria, the gate agent can do whatever they want and claim ignorance and no passenger can prove otherwise.