The teacher explains that you have to write a proposal to demonstrate your clarity of thinking around the chosen topic and that there is enough material to warrant your research. This is a no-brainer for you. You've already done the research. You write your thesis statement and drop a bunch of titles you've already read into your annotated bibliography and you're good to go. You sit down with your teacher to go over the proposal and you are careful to not say too much for fear that he will catch on that you've already done all of this. The meeting goes well. You tweak the thesis statement a bit, but not so much as to make you do more work.
You know that this will be easy to write so you decide to get it out of the way quickly. You write the paper over the first weekend of the unit. You've got lots of time before it's due so you tuck it away somewhere safe until the teacher asks for everything to be handed in. You don't have to spend any more time thinking about that!
On one level, this is disappointing. What a lost opportunity for learning. The student could have taken advantage of the opportunity to learn more about the topic that they clearly care about, but they opted out.
However, depending on their situation, this may have been a very appropriate decision. They already have a good knowledge of what the course is asking them to learn. They can easily demonstrate that through writing a competent essay. If they have other pressing demands on their time, especially if they're related to other learning that they might be struggling with more in another class, maybe the opportunity to focus elsewhere for a bit is a wise use of time and mental resources. It is not what we, as teachers, might wish for the student, but it might still be in the student's best interest.