Let's say you come home from buying groceries and ask yourself what to do. Notice how in this case you will never find yourself in an @computer context. You will need to scan your contexts and decide you can turn on your computer to put yourself in the @computer context. This shows that contexts aren't something that is always there but something you need to get yourself into. As such the context isn't highlighted by the system (and it couldn't be).
If you decide to sit behind the computer and do more next actions on it then either you find the actions quickly or not. In the latter case you decide that it's better to create a @computer context.
You misunderstood me. The context is always there, as soon as you declare a NA it also has a context, regardless if you write it down or not. Your NA "google for something" has the @computer or @web context, regardless of the name of your list. If you write said NA on a @computer list, than the context is highlighted by the system. If you write said NA on a all-context NA-list or into the calendar, than the system doesn't highlight the context. You than have to "see" the context while scanning your list or the calendar respectively.