Many places still have free camping in the USA but many do not … (add)
In UL publications, the term "stealth camping" is used for camping in areas not frequented by backcountry campers to avoid habituated bears and other wildlife. Mostly by avoiding designated campsites. Depending on the agency, this may not be entirely legal, though most books are a little dubious on this. California's Desolation Wilderness was heavily patrolled and I saw a large family, plus a couple individuals, go cross-country to avoid the ranger.
Otherwise, most non-fee BLM and USFS areas could care less but most have a 14-day limit unless it's a popular destination; if very popular, the agency enforces a daily fee for camping and/or parking. Then it just depends on the frequency of ranger and law enforcement patrols. Closer to larger cities there may be more restrictions to limit usage. Fine line between limiting impact vs. getting visitors to use the local hotels. Also in the US, there's the potential for crime closer to roadways.
While I agree with using pricing to limit impact, I disagree with it during "off-season" or for unpopular areas. In the land of the free, there needs to be some provision for a person to go on a thru-hike or cross-country bike trip IMO.
Also watch and respect private property lines. Even corporate holdings have local overseers who will apparently point a shotgun at trepassers while they await the sheriff to arrive to issue a ticket (around $100 USD according to a buddy of mine who experienced this). OTOH some corporations don't mind camping on their parking lots (think Walmart unless they changed their policy) ….
…. and then there's the entire "stealthing" movement in the US (Google it – people who convert cargo vans, pickups, and even cars for long-term living, usually for temporary economic reasons). No experience with it, though I've seen stealthers under underpasses in Los Angeles (… foggy windows, same location everyday). Desert SW, I've seen large camps under freeway bridges in dry creek beds.