If you only look at income tax brackets, that might explain your confusion because income tax is only one type of tax. On the other hand, sales tax tends to impact lower income people more than wealthy people because even though everyone may pay the same 5% sales tax, if you kept a ledger of all the sales tax everyone paid over the course of a year and compared it to everyone’s income, you’d find that as a percent of their income, lower income people paid more than wealthy people, which is why this article is saying some states’ tax structures are regressive, because when you look at the whole picture, not just income taxes but all types of taxes, the states that are “upside-down” tend to rely more heavily on sales taxes than income taxes to raise money.

The report published by the Institute of Tax and Economic Policy (which the article references) has more details if you’re interested enough to dig in further.

There is no cutoff, but:

• we have increasing brackets to a point, then they stop increasing , which can make upper middle the highest income tax payers
• income tax is only on “regular income” like salary. Wealthier are more likely to get their money as other types of income, taxed at different amounts, or play accounting shenanigans to shelter their money from taxes.
• sales and excise taxes proportionally affect people with less income more, since a much higher proportion of their income goes to necessities
• there’s a tendency to try to correct the regressiveness through greater complication, like my state does, but then are the less well off able to handle the extra paperwork to benefit?

Edit…

• social security and Medicare taxes are only in the first x amount of income so are very regressive, although the payback is also less beneficial to the well off so I’m not sure how to count these