It's not necessarily true at all.

Volunteer work can be full of really shitty and awful people who justify their shittiness based of their 'good intentions'.

I have volunteered my entire life and seen many orgs fail/fall apart due to shitty people getting involved and cannibalizing the org for their own selfish ends, or wanting to turn it into a social club first and a volunteer org second/third.

It can be rewarding, but a lot of orgs are full of very catty insular people who don't want you to be part of their group unless you look/talk/walk like they do, and you tow whatever 'party' line they believe in, even if it's shitty.

Someone volunteering doesn't make them automatically a good or a nice person. That's the smokescreen shitty/selfish people use though to justify their crappy behaviour in these orgs.