That's just the rules. For an RMD you can actually request no withholding. But, for anything more they MUST withhold.
Yes, it's effectively "adjusted later". No piece of income is taxed individually. All your income, including pension plan distributions, is added together at tax filing time. The tax is calculated on that total (after adjustments, deductions, exemptions and credits). Then withholding is subtracted to determine if you owe more or get a refund.
