That may be true - unlikely to get a big fine for a first offense. I live in Colorado and when I first started flying a few years ago, right after getting my Part 107 certificate, I flew in a city-owned outdoor recreation area after checking my apps and online for any regulations and not seeing anything prohibitive.There were no signs posted there. I checked specifically for that.
"Ignorance of the law is no excuse" is more of an eastern thing. If there's nothing posted, you'll get a warning if you accidentally violate some ordinance.
Repeat offenders would get a lot less slack, of course.
TCS
A city LEO was in the parking lot after I had flown and packed up everything and asked me what I was doing. I gleefully told him what I was doing and he said no drones were allowed in that area and said not to fly there again. Fortunately, I knew him personally, and he was very pleasant about it all.
The park still hasn’t installed any no drone zone signs, but they have since posted the regulation on their website.
Not everyone west of the Mississippi has the mindset of “we don't assume that you need to get permission to do things.”