don't complain later when your child is one of the many, many students who rushes or ignores their independent study and fails to progress as a result. You would have brought this on yourself with what you modelled to them
I strongly disagree with this. I don't think that objecting to one piece of homework given to a 5yo constitutes modelling a bad attitude to work. The OP mentioned that her DS already had a reading book, and some Maths work to complete that night, and presumably she would have been supporting him in this.
IMO what defines children's attitude to independent study later in life is being enthusiastic about learning when they are young. At age 5 there is a very fine balance between providing enough stimulation and making children feel over-loaded and demoralised.
My DS is 5 (summer born) and has also gone into Y1 this year. He spends a lot of time feeling like he cannot manage the work at school - writing a complete sentence takes him a long time and he would find writing a whole letter challenging. OTOH he is a very bright and happy boy and I have no concerns other than that he is a reluctant writer. I am very aware of this and try to make writing a positive activity at every opportunity at home (e.g. writing birthday cards for friends etc). However, trying to get him to finish a letter after school would be met with resistance as he would be tired, grumpy, and struggle to complete this. I fail to see how this would be a helpful lesson.
As I said, I think the most important thing at age 5 is that children should feel that learning is enjoyable and is something that they can succeed in.
I don't think it's appropriate for a teacher to suggest a 5yo completes a task 'in bed' as the teacher had asked in the OP. If there isn't time in school for the letter to be completed, then not enough time was allocated for the task.
In many countries in the world children this age would not be in formal education at all and I think it's a shame to impose unreasonable demands so young.
Pupils of secondary school age who rush or ignore independent study are not necessarily the 5 year-olds whose parents objected to excessive homework. As I said, I think if children are taught at home to see the value of independent thinking, reading and writing etc in fun contexts they will see these activities as worth pursuing later on.
I know anecdotes have little value but I had no homework at all at primary school and just read books for pleasure, encouraged by my parents. This had no influence at all on my attitude to homework later on - when I first received homework at secondary school I was the right age to understand its value and always completed it. Similarly my brother did little or no independent study after the same upbringing.
I think teachers and parents should stop blaming each other for students' negative attitudes to academic work, in or out of school, and try to work together to demonstrate that learning can be valuable - this does not include overloading 5 year-olds with homework.