"Schools should be allocating books with a teaching purpose in mind ... skills to develop or strengthen."
Absolutely. And teachers should be having that meaningful conversatiomn when a parent feels there is a gap between the 'book band' [I won't use the word 'scheme', because our book bands are a real mix of different schemes, real books etc] and 'reads at home for pleasure' books. There is, certainly when I have those conversations with parents, a reason for the gap - it often comes down to what a parent means by 'reading' [essentially decoding the words] and what a teacher might mean by 'developing a particular reading skill' [e.g. inference] - but I should be able to explain that reason, not simply state that it is how we do it.
However, there are, very rarely, some parents who simply have lost faith / belief in the school / the education system / the teacher, and however lengthy and rational and evidence-based the discussion, will simply not accept any reason for the use of a specific book band level. If that type of communication breakdown occurs, it is really difficult, because both parties are left with a sense of 'grievance' - the teacher that professional, evidence-based judgment is not accepted, the parent that their view has been challenged, as they see it unreasonably. Those are, luckily, rare interactions - I can think of 1 over the last few years - but they are so difficult that they stick in the mind.