The issue for some is that the State of California has banned certain chemicals that used to be popular for many purposes, including woodworking. Within that, the South Coast Air Quality Management District (SCAQMD) has imposed even further restrictions, banning additional chemicals from most of Los Angeles, Orange, Riverside, and San Bernardino Counties. They add to the list of banned chemicals from time to time, too.
San Diego and Ventura Counties are not affected by the SCAQMD restrictions, but they are subject to California rules.
Banned chemicals in most of Southern California include true mineral spirits. Before the bans, paint thinner was mineral spirits. I don't know what is in today's "paint thinner" here in So Cal, but it isn't mineral spirits.
Some companies reformulated their products for the Southern California market. I read that Waterlox has two different formulations - one for So Cal, and one for everywhere else. And Waterlox says that you better not mix the two or thin the two in the same way. You need the right thinner for your kind of Waterlox. (I have looked for Waterlox around here, but have yet to find it. Everyone says, "Yeah, we used to carry it, before the ban.")
People talk about "VOC"s here. That stands for Volatile Organic Compounds. I assume that mineral spirits count as a VOC. For example, woodworking finishes that you buy can't exceed a certain percentage of VOCs or they are banned. Of course, people who buy them want to thin them to deal with the issues that arise. If you think about it, thinning turns it into a banned product. Doh!