Confused on Taxing of Roth Distribution

I took a ROTH IRA distribution this year of of $14,255.  I had made previous contributions and distributions over the past 9 years that totaled $12,690 so I knew I would owe tax (and a penalty) on the difference but I don't understand why the entire $14,255 is being considered taxable income in general on my taxes.  I paid tax on the contributions before I put them in the ROTH so why am I paying taxes on the entire withdrawal and not just the amount that I am drawing from the growth?  That seems like double taxation to me.  Thanks!

 

Brian