For years, ever since ds1 was a baby, I was a serial volunteer - the local NCT committee, the playgroup, helping with PTA events at their primary school, being a parent governor, working in ds1's senior school library, the art class I attended's committee, the street's residents' association and my choir's committee.
I have never met, in person, the attitude that the OP outlines - about busybodies etc - but have definitely met the constant criticism from people who were happy to bitch from the sidelines, but never, ever stuck their hand in the air when we needed more help with things.
I have, over the years, run myself into the ground, and got very stressed over responsibilities I had taken on - but have felt very torn - I didn't want to stop doing things, when I knew that no-one else would be taking it on, and that, by stepping back, I would be putting even more of a load on the shoulders of the few people who were prepared to volunteer for things - but I have had to step back from things for my own mental health.
Now I am on only one committee, that doesn't involve a great deal of work or responsibility, and I feel so much lighter as a result.
I am willing to bet that the people who carp and criticise the few willing volunteers and their efforts couldn't care less about the effect this has on them - nor do they realise or appreciate the results of people's work and effort, that benefits them or their children. Sadly I suspect that, as @TeenTimesTwo says, it takes the loss of these benefits, for people to start to appreciate the volunteers.