I agree with you, but I also had instructors who demanded the same thing: essays as arguments, drawing on evidence and analysis to back up a thesis.
An essay that is simply a description of the source material, or a listing of pros and cons with no actual thesis, is a totally different endeavor, and one that has much less value as method of instruction or assessment.
Restating or paraphrasing a widely accepted thesis statement isn't very educational? When your freshman class of 200 students is writing essays on a handful of topics, the same ones every year, that every other school touches on, there's not going to much in the way of originality.
Then again I guess the same argument can be made for maths but no one complains that we're teaching to memorize and repeat the same steps of various proofs one learns in 1st year.
An essay that is simply a description of the source material, or a listing of pros and cons with no actual thesis, is a totally different endeavor, and one that has much less value as method of instruction or assessment.