I think payroll taxes should be included because it is actual money coming out of the pockets of the people who pay them. If somebody makes 25,000 in a year, and 2,000 comes out of their paycheck due to payroll taxes, that is fairly significant. It would be perverse to point a finger at this person and say "this person doesn't pay federal income taxes!" as if they're some kind of freeloader, when they felt 2000 dollars come out of their pockets that year. It's a tax on their income, by the federal government, based on a percentage of their income... sounds like an income tax to me, and it feels like an income tax when it comes out of the check.
The fact that it's earmarked for a specific benefit instead of going to the general fund seems to be a needless distinction.
This isn't directly related to this thread, but I feel it's close enough to be worth a mention. If people want to get angry that the federal income tax is too progressive, and that the other 50% should "pay something," consider this. Even at a lower percentage of income, the day-to-day impact on the life of a poor person of taxes can often be greater than a higher tax on a rich person. Even if we taxed every dollar of somebody who makes 1,000,000 per year at 90%, they'd still clear 100,000 per year. If somebody here wants to say a person cannot survive on clearing 100k per year, I'd like to hear it.
Contrast that with somebody who makes 15,000 per year. At that amount, a person will be barely struggling to make ends meet, and is probably risking homelessness at any time. Taxing them at a mere 2% per year would take 300 dollars away from them. That could be the difference between eating and not eating for 2 months. I'd argue that in this scenario, the person making 15,000 per year is being taxed much more harshly than the person making 1,000,000 per year, despite one being 2% and the other being 90%