If someone is giving me fiat money, I know that I can spend that money as easliy as I got it. The same thing should be with Bitcoin, so how are we going to handle with this issue?
Not necessarily... If someone gave you a pile of fiat that was splattered with some bright colored dye or paint, such as what happens when a bank robber flees the scene of the crime and opens their bag of ill-gotten bills only to find an exploding dye canister going off in their face and all over the bills, would you really expect a bank or merchant to accept those "tainted" bills? This is the fiat equivalent to "tainted" Bitcoin.
I haven't personally decided if I support the idea of "tainting" bitcoin yet, but this would be an example of the real-world equivalent.
I have travelled to over 40 countries in my life and I have exchanged fiat curenncy to goods or another currency for probalby (guessing here) more than 10 000 times, everytime this transactions has gone smoothly, maybe I have been a lucky guy, buy I think we all have similar experinces with fiat.