We're having this issue too. My daughter loves reading chapter books and is bored by her school reading scheme books. The teacher says she needs to work through all stages of the latter, because she 'probably' isn't understanding all the words she comes across in the chapter books, and that comprehension is the most important thing.
My line is that, in the long run, comprehension is vital. But we are a way off needing to worry about absolute comprehension (she's year 1). And besides, in two or three years' time her vocab will be far, far better than it would be if she stuck with the stuff she can already understand. (I had a reading age of 18 at age 9, whatever that means.) In the meantime, she is happy to ask about words she doesn't know if they interrupt her enjoyment of the story.
And surely that is real learning: voluntarily trying things that are just beyond our capabilities (as long as this isn't forced and doesn't cause stress or worry)? I can see that some children would be put off reading by being given texts that are too hard. But I really don't think it should be too difficult for a teacher to work out the best approach for each child in a class and to vary the approach.
This seems to be another example of schools forgetting what their real purpose is. They should be educating in the purest sense, and encouraging children to be what's that buzz phrase independent learners. Instead they are fitting children into patterns, ticking boxes and following formulae. As usual. Makes me cross.
Of course, the last thing I want is for my child to start thinking that her school is irrelevant and she doesn't need to do what the teacher says so ... I hide what I think, we dutifully read the scheme books together, quickly and then we get on to the good stuff (which I also report in her reading diary so the teacher can see what we're doing at home)!