What do you think this Acumen thing is plotting? It seems too easy to just imprison us and let us live comfortably in a little city. There's gotta be a catch, right?
[ The Catch is what worries Usopp. If it were a normal prison, there'd just be an execution sentence - that's scary enough. But there isn't, so whatever is in store has to be a fate worse than death, by Usopp's logic. ]