Am I misunderstanding something, or are tax guidelines designed to punish married people?

I’m filling out my first W-4 as a married person, and I’m surprised by some of the things I’ve never noticed before. A single person with two jobs does not have to file for additional withholding unless he makes over 40k. However, if married people both work, they have to file for additional withholding if their combined incomes are greater than 10k. So, a single person is fine up to 40k, but two married people that are living off the same money are only good up to 10k. The tables and instructions on the back indicate that even if their combined income is only $10,002, if it is split evenly, they have to pay an extra $560/year in taxes. How does that make sense?