Why do I still have to pay into Social Security? I retired and started receiving benefits 16 months ago.?

I mean my life long income, up til 16 months ago, was used to calculate my monthly Social Security check amount. And Soc Sec Admin penalizes me if I make more than $14,160 a year, until I turn 66. So why do I, and my employer, have to pay Social Security on my $14,160? They are just making it harder on Seniors who are on 'fixed' incomes.