You are not paying tax on more income. You are paying tax at a higher rate on part or all of your interest income. You pay an additional 3.8% tax on part or all of your interest income. On Form 8960 your net investment income is on line 12. The amount of income that you pay the additional 3.8% tax on is on line 16, and it cannot be more than your net investment income on line 12. You are not paying tax at your regular tax rate on the line 16 amount. You are paying 3.8% additional tax on it.
Why do you have to pay the additional tax? Because the tax law says you have to.
You said your total income is over $200,000. That means your income is higher than 94% of the population. Frankly, you are not going to find a lot of people who think there's anything wrong with your having to pay a little more tax at that income level.
