I think they restricted it to 50% because it's looking at federal taxation (payroll, local and state level gets much more complicated), and ~43% of Americans don't pay any federal tax or receive a rebate. Many of these people are elderly, low income with children, unemployed, etc. But including them when looking at the impact federal taxes have would be incorrect.
> think they restricted it to 50% because it's looking at federal taxation (payroll, local and state level gets much more complicated),
Federal payroll tax is part of federal tax (as are federal excise and consumption taxes).
> and ~43% of Americans don't pay any federal tax
As well as being kind of dated and thus quite likely not accurate, this number os for federal income tax, not the more general category of federal tax (or even “federal taxes on income”, which includes payroll tax.)
[0] https://www.statista.com/statistics/242138/percentages-of-us...