That's true for the State of California, AFAIK. But in places (or some type of organizations) with just cause instead of at will, firing you for moonlighting would be illegal if it didn't conflict with an actual required duty.
I was always a little puzzled by this concept and I think it gets more silly every decade. How can someone routinely spend money on goods given how insanely cheap goods have become?
There's maybe a small percentage of the population addicted to buying brands beyond what they could possibly use, but most people run out of the ability to buy a significant amount of stuff every year. I.e. even a thousand a month habit is insane to maintain and nothing compared to bad housing choices.