It does sound like the teacher is disorganized; a class letter about behavior sounds a quite bizarre too. I can tell you, though, there are a few reasons why a teacher may be hesitant to have a parent in the room often:
1) A chatty parent who wants to either converse with/ or have a mini-conference about their child during "volunteer" time.
2) A parent whose child will be all over them during "volunteer" time, without the parent telling the child to get back to work, etc. The teacher doesn't want to do this and seem like a meanie, overstepping the parent's authority. This can be really disruptive.
3)

WORST of all: the parent who, during "volunteer" time, makes observations of every other child in the class, and proceeds to spread gossip among othere parents about whose child is smart, stupid, dirty, poor, snobby, or badly behaved. Then, in the following year, uses these observations to tell the next teacher that his/her child can not sit by ____ for reason _____ .
I am not at all suggesting that you fall in these categories, just giving some defense as to why some teachers are hesitant to even have parents in the room.
Also, many schools have a rule regarding parents not grading/checking student work. I would not feel comfortable having a parent volunteer do any of my paperwork of any kind. You never know what information they might come across regarding another child's personal information.
Things that I love to have parent volunteers for (perhaps you can offer these up as suggestions to your child's teacher):
-read to a small group of children
-cut out things that have been laminated
-cut out shapes, etc for a craft project
-tear out, sort, or collate upcoming workbook pages
-laminate things, make die cut letters, etc in the work room
-work on letters, numbers, sight words, etc with struggling students and record their progress (This is not for the gossipy parents, though.)
I hope this teacher makes you feel more welcome in getting involved with your child's school.