50% aren't paying federal income taxes because their income is so LOW. As wages continue to stagnate, fewer people cross the line where they pay taxes. Don't understand wage stagnation? Don't think it's happening? My GF is in the food business. She was a restaurant manager in the early 2000's and started at 36k / yr. She found a position the other day similar to that one, with about twice the responsibility and hours, guess the pay? Yup, 36k. All the jobs she finds similar to that one from the early 2000's are all around the same number. Now what costs have gone up since then? Everything. As long as this continues nothing is going to get better.
This is why I despise the "trickle down" arguments made by the GOP and TEA parties. It used to work, but just does not anymore. The money goes up, and is never paid back down, at least not in the amounts (by percentage) that they used to. People need to spend more money to get out of this, but they have to earn more to start. No amount of education can escape teh fact that people are being paid at rates from 10, in some cases 20 years ago.