I may be misinformed, but I believe WFAS is actually based on the original Mount & Blade rather than Warband. Which may explain why it doesn't have many features of Warband. I suspect it may also be using "Well it's based on a book" as an excuse for not including those features, on the grounds of "well it wasn't in the book so why would we ever include it?"
Presumably WFAS plans to stand out in terms of story (which I haven't played into, so I couldn't say how much it succeeds or fails there) and novelty (guns!) rather than sandboxiness. And while that may have been the intent from the beginning with WFAS, it can seem to lack the free-forming panache of Warband. There are things I like about WFAS (guns, caravans, mercenary camps (yes, believe it or not, I like the idea behind that mechanic, if not the implementation)), but it can also feel "flatter" for lack of open-ended structure.