No it isn't. Not at this age. I go on the preschool field trips because they're fun and I'd really rather drive my own children when they're that young. I go on the school field trips again, because they're fun and its something I get to do with my kid. I see no reason why a parent shouldn't go along.
I go because I do not feel comfortable letting my child go with a school full of kids to a PUBLIC place full of other schools full of kids, even if there are chapereons. Now, my DD goes places with friends and their parents but the adult/child ratio is much lower. I am not overprotective in my eyes or many others. Maybe some parents are not protective enough? I am just saying that no one (especially the school) should be making judgemets on a parent's parenting style.