I’m with Frink on this. The Emperor is making it out as if his best legion of troops await them at said location, so his VERY best troops should be there on standby. Also, this means the Emperor should had already informed his troops ahead of time that a Rebel strike team was heading to the Shield Generator. Trap, right?
Yes it is a trap. But he doesn’t know where exactly they will strike. And as I already pointed out the obvious, the complex is huge and the legion had to be dispersed to make a trap effective at each of many potential infiltration points. Or should he just put everyone at one door and hope for the best? It doesn’t take a general to see these obvious things. All it takes is common sense.