I think because most coffee including decaffeinated has a lot of chemicals in it due to how they process it.
I am not sure about coffee beans and you grinding your own though. I think a lot of the coffee is treated chemically as it goes through harvesting too. I might be wrong but everything else is so why not coffee too?
I would check out a health food store or organic grocer and ask them if they have any recommendations. Like all the other things you can now purchase I would think some coffee producers/growers would have jumped on this bandwagon as well and started producing healthier versions.
Another reason might be is that coffee is a natural diuretic and it might also flush out some important electrolytes along with the bad stuff that you are trying to detox with. Also, maybe it has something to do with your internal organs and how they process coffee.