They are taking the risk of getting questioned by the police and possibly doing time. That's the risk they are taking for buying these tainted coins.
Say that I received 1 BTC from ISIS. I send this bitcoin to whirlpool. It's getting mixed 'til eternity. Please explain me the risk from selling coinjoined coins. I have no direct relation with ISIS from a blockchain perspective, and chain analysis will simply flag it as "taint" because of its inability to track it, just as with XMR.XMR is 100% fungible, unlike bitcoin. That's why the government doesn't like it and centralized exchanges are delisting it.
Say you received 1 btc from ISIS and sent it to whirlpool... when the coin is mixed, the authority won't be able to know if your coins came from ISIS or not. This time, you are bearing the risk of using mixed coins. You exchanged one risk for another. The gov don't like that too.
You brought decentralized solutions to the discussion but last time I checked, bisq was a ghost town and exchanges like eXch aren't really decentralized.
I love how we transitioned from "certain coins can be traded for a 50% discount" to "b-b-but Bisq lacks liquidity". You know what it brings to mind? "Bitcoin holds no value because no vendors accept it". 
If there's insufficient liquidity, make an offer. As for eXch being centralized, yes, it can be shut down. But, is it currently treating all coins equally? Yes. Are there more swap services that stick to the same notion? Yes. As long as there's just one, Bitcoin is fungible.
Certain coins can be traded for a 50% discount indeed. Do you deny it? Are you saying it never happened? Do you claim people will never buy bad coins for a discount?
Bisq's liquidity is another topic. It certainly lacks liquidity. That's another fact. Do you deny that too?
If nobody accepted bitcoin, bitcoin wouldn't have a price. You are selling 1 coin for a million dollars. You are the only seller on the market. There are no buyers. What's the worth of that coin? Zero or one million dollars?
Every other single person out there who's using dex's and how many are there of those people actually? Can you give me a number? And compare it to the people that use centralized exchanges maybe? What is the rate? 1 to 100? 1 to 1000? 1 to 10.000? There aren't as many people that use dex's. as you think there are.
"How many people out there use Bitcoin as currency, 1 to 100, 1 to 1.000, 1 to 10.000? It's not money!" 
As for Andreas, I don't completely agree with him on this matter. He once told its viewers to send coinjoined coins to themselves for k times so that "taint" can disappear and their centralized exchange accept their private coins, which is completely moronic, and due to the nature of "taint", is not guaranteed to work.
You said people use dex and trade their tainted coins all the time and I asked how many people? I see there are only ~250 offers for FIAT/BTC pair. The volume on bisq is indeed booming /s
Andreas knows his shit, definitely more than you. :/