Do-Gooders

I've noticed that reasonably often, whenever someone is disparaging a cause, or a progressive person, or something a bit lefty, people throw around the term 'do-gooder' like an insult - 'Aaah all these bloody do-gooders going on about crap all the time!' This weirds me out because... are they saying doing good things is... bad? And that, conversely it's good... to not be good? Okay I'm confused.