The public courses in Los Angeles are so full of people practicing during the day, you can barely find a spot to warm up prior to your round.
In cases like that it would make sense for a course to post rules/signage that you have to purchase a round or range balls to have access to the seemingly "free" practice areas.
I live in SE Wisconsin where overcrowding is never a problem. There are a couple of public courses near me at which I often practice, and sometimes it's just my putting so I don't pay anything. I also play these courses pretty regularly and know the staff. I would be shocked if they told me I had to buy range balls or be paying for a round that day to use the practice green.
I will also add that I'm appreciative of this privilege and will help clean up the practice green if someone has sprayed 50 range balls all over and left them. I also am NOT the player who sprays balls all over during practice (in fact I generally only practice putting with one ball at a time).