The Stormcloaks do suck, and they are unwittingly playing right into the Thalmor's hands, but the Empire are also getting completely punked by the Thalmor. Maybe more embarrassingly. Ulfric is trying to fight them but he's too stupid to realize he's giving them exactly what they want, but the Empire thinks that in order to accomplish their long term goal of defeating the Thalmor they need to give the Thalmor everything they want in the short term and enough internal access to the Empire to completely undermine it from within.
Well now, that's subjective.
I mean, it's the slightly longer route, but if Ulfric wanted to hit a spot along the way, such as the Shrine of Talos or the should be Stormcloak controlled Fort Amol, or if he perhaps wanted to avoid a spot on the more direct route because his spies suspected an ambush, it's not that much of a detour.
People don't always take the most obvious and direct route.
If you run through on an Imperial Civil War playthrough you clear it out ahead of the Battle of Windhelm and the Stormcloaks have retaken it, so it's presumably a Stormcloak controlled fort that got temporarily taken over by brigands before being reclaimed.