I don't know about the rules in California, but in most places you're not supposed to go back to work for at least a week if you've tested positive regardless of your symptoms. This is to prevent further spread, of course.
It's quite likely that most of the workers in question aren't worse off than with a "normal" flu, but they're still going to be away from work for a while.
Someone defined the difference as "if you think you're going to die, and feel that dying might be better than staying sick - then you have the flu. Otherwise you have a cold.".
I don't think these people can afford to take that week off. Most of these people are paid by the hour. If you aren't working, you aren't making money to pay bills and get groceries.
I am making some assumptions here, but I would venture to guess a lot of these people just fake it and grind through it just to stay employed.
It's quite likely that most of the workers in question aren't worse off than with a "normal" flu, but they're still going to be away from work for a while.