They are not the hacker, they haven't stolen but they have exchanged something with him and received that input. Won't it get them some hassle?
It might, but the hassle comes entirely from the fact that the centralized platform they are using fundamentally does not understand how bitcoin works and is attacking bitcoin by perpetuating the nonsense that some bitcoin are worth less than other bitcoin. The hassle does not come at all from the bitcoin in question being intrinsically dirty, tainted, or any different to any other bitcoin. And as I said above, if you are ever faced with such a scenario, the correct answer is not to accept that the bitcoin you are trying to spend are somehow less valuable, but rather to find a different service which does not make such ridiculous arbitrary judgements.
Well, the problem is with centralized services only.
Precisely. Just use bitcoin peer to peer without going through a centralized third party - as it was always intended to be used in the first place - and any problems with taint or other made up nonsense completely disappear.