I havent lived in LA in a while, but I do visit frequently, and all I can say is how the f does anyone speed, its a parking lot (at least west La and the valley is) all the time. I was driving back from a party in the valley towards Santa Monica at 3:30am and it was stop and go traffic on the 405.
That's the FourOFu!@#d for ya. And sounds like you've got the idea - people complain about getting a speeding ticket out here, but selectively leave out the parts of the story that include the school zone violation, residential area violation, driving down the emergency shoulder on the freeway, and while talking on your cell phone at the same time violations in adition to the speeding charge. If the freeway is moving, no traffic, everyone on it is doing 70-80, and unless you're doing 90+ or soemthing else stupid you don't get a ticket. Same with commercial streets, off-hours, non-packed 35-zone they'll let ya get away with 45.
Also, there is a profound difference in attitude and "styles" between LA County Sherrifs and Los Angeles (Municipal) PD. Most Sherrifs are (not nice), don't know why, they like being the law and slapping it in people's faces... I personaly suspect it has to do with the Sheriff's department requiring deputies to first work a year or two in the jails. LAPD will be more understanding and often lets ya go with a warning if you weren't doing something too stupid and pass the "attitude/respectfulness test".