Journalist here: Yes I do think that journalists often work for free. Many, many journalists work for free until they "make it". It's sadly such a competitive field that it's the only way to get paid work down the line.
Regardless though, the most-critical local reporting I know of is funded without advertisement or paywalls. Both newsroom and freelance journalism funding happens through grants. Funny enough, the orgs that I'm never able to get freelance work through are orgs that have paywalls and significant ads. Lots of gatekeeping when you have advertisers to appease.
Do you think that journalists just work for free?