In the Warhammer 40k universe, Commissars are indeed saluted by the soldiers of the Astra Militarum (Imperial Guard). This is because:
They Hold Significant Authority: While not part of the regular Imperial Guard chain of command, Commissars are political officers of the elite Officio Prefectus who oversee the regiments, ensuring morale, discipline, and loyalty to the Emperor.
Power to Execute: They possess the authority to summarily execute any soldier, including officers, deemed incompetent, cowardly, or showing signs of disloyalty. This absolute power commands respect and, perhaps, a healthy dose of fear.
Symbol of Imperial Authority: Commissars represent the unwavering will of the Emperor and the Imperium within the ranks of the Guard, and saluting them is a sign of respect for that authority.
Maintaining Order: Their presence and the discipline they enforce are crucial for maintaining the cohesion and fighting spirit of the vast Imperial Guard forces.
Therefore, the salute signifies a combination of military courtesy, recognition of their unique authority, and an acknowledgment of the power they wield within the Astra Militarum.
https://youtu.be/YkIrDq-I8Qs