This is an exaggeration, at least for the US. Yes, it's true that taxes will take a big chunk out of your $10k / month, but they'll take a pretty big chunk out of your $120k / yr salary, too. The only real tax "savings" you get by being employed is that your employer pays the other half of your Social security and medicare taxes (12.4% total for 2012), which means that you'll have to pay an "extra" 6.2% if you're self-employed. And most of that is SS, which caps at $106k or something. Couple this with the fact that self-employed folks have a variety of deductions that employees usually don't get (home office, special retirement options, lots of business expenses), and it starts to look a little more even.
It's true that you don't get paid time off, but I can tell you from experience that a decent and determined freelance programmer can easily make $10k / month and leave plenty of time for relaxation.
The biggest thing most self-employed folks have to deal with that they wouldn't normally is health insurance, but if you're young and healthy, decent insurance is a few hundred bucks a month, at most. And you can deduct the premiums. If you're not young and/or not healthy...that's another story.
Bottom line, if you want to match a $120k / yr job as a freelancer, you need to make something like $12k - $14k / month.
Source: personal experience and a fascination with legal tax avoidance strategies.