I don't really know Alloran's character that well, but here's the only explanation I can think of that makes sense with the Hork-Bajir guard.
Maybe Alloran wants to live so badly that he won't even try escaping and risk being killed? Or he just values the two hours of non-infestation every three days so he doesn't want to escape, fail, and have to give it up by being rendered unconscious every time after when Visser 3 feeds.
Since Visser 3 is a Yeerk, he knows Alloran's thoughts so he knows Alloran won't try to escape. In that case Visser 3 wouldn't bother sedating Alloran or using a Ramonite box or portable Kandrona. The Hork-Bajir are just in case.
The only thing that would counter that theory is when Alloran asks Ax to kill him in Book 9. Was that a KASU, or was the Hork-Bajir guard as opposed to some other method the KASU?