When the amount needed to get past FS is higher the incentive to work is less. It is much easier to get to a salary of $30k than it is to get to a salary of $60k. If 90% or more of earnings until $60k do not benefit the person then what is the incentive to work?
To be honest, that actually makes sense. (90% is an over exaggeration, the regular guy on programs loses about 50-60% of his income, until the insurance cliff comes.)
But all truth be told, I never really cared if there was an economics argument to be made for the proposal.
I still don't want to see tens of thousand of my brethren, being thrown into poverty.