The "captive" term isn't really meant to be taken literally. Obviously, there are many choices available from the hubs via connections on other airlines. It's just that most people (aka, non-FT's) simply regard air travel as a means to get from point A to point B. They look for the quickest and most convenient routing which tends to be non-stops, even though they may also be the most expensive options.
Similarly, when people talk about fortress hubs, they don't mean that defensive walls have built around tha airport and soldiers stationed there.