I agree with you 100%.
I would like for us students to be able to write whatever our heart desires. What bothers me sometimes is when we have to write a ton of essays on the short stories we read; such as writing about the theme of the story, symbolism within the story, and so forth.
However, I can see why teachers would rather assign the writing assignments. It's to help us improve our writing and in a way, reading skills. For me, when I had English class my first essay was horrible and lacked examples from the text. As my teacher kept assigning essays, I found myself improving little by little.
Overall though, I still do think teachers should allow us to pick out a topic to write about. The closest time where that happened was when my teacher gave 3 different topics and we got to write about one out of the three. That's somewhat close to being able to pick our own topic.. but at the same time, not really haha.