Explaining the value of what you are showing should always go towards the start. Else, why would anyone bother with the rest?
Giving such a blanket "responsibility" to the author at all is just such a bummer! I say let them do whatever they want, there is always more than one way to express oneself. Someone who was never taught to write a clear thesis in the first paragraph for whatever reason doesn't inherently have less to say.
Never heard of Abstract section? First semester on a college or last year on high school.
Haven’t you dramatically overstated your case? Many expositions do not contain an explanation of their value at all. Works of fiction are a good example, and there are many many others. Often it’s the responsibility of the readers & reviewers to decide on questions like value.