Flat tax proposals are nowhere near 50%.  The idea is that there would be a flat tax (i.e. 25%), with plans to mitigate the taxes for the poor by offering tax credits which will offset (in whole or in part) the taxes owed.

Most people would pay less than they do today, and those that don't pay anything today likely wouldn't pay anything then either (if they were legitimately low income, for example).

On a principals basis, our current system is unfair.  It is "unfair" to take more (as a percentage) from those with more money, as we do today.