I feel I gave some valid reasons as to why a teacher may be reluctant to have a parent observer in the classroom.
Are you a teacher yourself, or have any experience of teaching? If your child did display what the teacher felt was inappropriate behvaiour, would you be able to sit back and watch their interaction without feeling the need to jump in and give your opinion? Could you judge professionally whether the teacher was right in his/her decision to reprimand your son?
The level of poor behaviour tolerated within a classroom is always going to be far less than what most parents would tolerate. This is simply because there are 30 children in the class to manage. The teacher has to be thinking of all of them, not just one individual. What you may feel is high spirits may well be inappropriate for a classroom full of 30 children.
And I don't think it is teachers necessarily being defensive. Teachers have lots of observers in their rooms - support staff, student/trainee teachers, work experience, senior management team, head teacher, LEA inspectors, OFSTED inspectors, parent helpers, etc. But having a specific parent coming in with the sole aim of watching them and how they interact with their child feels somewhat too obtrusive.
I really am not sure where the benefit would be - I truely think the dynamics with you there in such a role wouldn't work. Your child is likely to react differently, the other children are and the teacher also may well, especially if they feel their every move and word is being judged in some way.
I think a better route would be to start with having the conversation/discussion with the teacher about your child's behaviour. Find out exactly what it is that is the problem and how the teacher is currently managing it. Ask how you can support your child through this process., and then take it from there.
Also, bear in mind that it may not be the teacher his/herself who makes the decision about who can and cannot come into their classroom, and in what form. Those decisions generally come from higher up, such as from the Head.