Why wouldn't they tap social security? It's their money they are getting back, and yes there is a sliding rate in that the higher the rate of income the less credit one receives toward benefit payouts on a per dollar basis, works with brackets like a progressive income tax in reverse. There's also a total income formula for income tax on benefits that specifies that up to 85% of social security benefits can be subject to taxation.
Social security will be withheld from an employee's wages at the rate of 4.2% (down from 6.2%) up to the social security wage limit of $110,100.
Publication 505 (2013), Tax Withholding and Estimated Tax
A person making $250k a year only pays into SS up to $110,100. The point of the OP is why the cap?
I also think it's ridiculous that SS and unemployment benefits can be taxed.
the amount withheld from your paychecks for SS is not taxed when you earn it. SS benefits are taxed only if your income is above a specified amount.
as to unemployment, you would pay taxes if you were working for the money, why not when it is being given to you?