Writing fanfiction most definitely isn't a passive consumerist activity. And reading it can be - but it doesn't have to be. When I read fanfiction, I am critically engaging the source material, something I do without fanfiction, though the lens of fanfiction helps with the mental processes by putting things into specifics. Take for example, the shows Buffy and Angel, a fandom I write extensively in (almost exclusively, at the moment). The shows have a lot of complexities, themes, etc. They can be just watched and enjoyed, and I do that with them to, but they also raise interesting questions, both 'big' questions (just how many lives must an immoral action save before it becomes moral? What measure is a non-human? What is human? etc, etc). But the show also has a lot of serious debate about characters, motives, mindsets, etc.
Is the Watcher's Council an evil institution, a corrupt one, a flawed organization trying its best? Was it always this way, or did it start one way and end up another? Not only is there serious engagement there, but it does again, bring back around to bigger issues about ends, means and such.
Is Angel morally culpable for what Angelus did?
Does Wesley really go insane in the times when he seems to, or is it instead that he is 'too sane', having lost the pleasant lies we tell ourselves that help us function, but make us all a little insane?
Etc, etc, etc.
I don't always think about these things when dealing with fanfiction, but no one always things about these things when reading anything - and I pity them if they do.
Fanfiction is many things to many people - its great practice in the sheer mechanics of writing, allowing someone to really get into how to write without having to worry about the issues of creating a world or characters or what have you. But plenty of fanfiction writers also create - they explore gaps in the world or the story, they get inside characters heads in ways the writers never did or never could (getting inside a characters head in a TV show, for example, isn't an easy thing to write). Its fun to play around with other people's worlds. Its a way to get feedback on your ability to write faster and sooner, and you know you'll have something of an audience to work with from the word go.
Is there an element of laziness? Yes but any genre of published fiction often uses crutches of various kinds just as much as fanfiction uses the original source material - literary norms, genre devices, character archetypes, thematic/mythic arcs, standard conceptions of various kinds of creatures, elements from RL history, cultures, languages, myths and so on.
Somewhat nonseriously, I've referred to writing and reading fanfiction as going beyond the Limits of Imagination - obviously, its not actually beyond those limits because people can imagine what you're reading, or you can imagine what it is you are writing. But in another way, it is going beyond the limits of imagination -certain kinds of stories in the world of fanfic could never be written with the same styles and manners as a work of original fiction, because fanfiction opens doors that just don't exist in orginal fiction, allows certain things original fiction just doesn't.
Most of the greatest works of the western canon (Illiad, Odyssey, Aenied, Shakespeare, etc) are essentially fanfiction, and in some cases, really fucking derivative fanfiction. Doesn't make them any less great in their own way.