One being a criminal forfeiture meaning you would have had to been convicted of a crime and purchasing bitcoins on a darknet is not criminal under any statute. The second is a civil forfeiture which requires that the property (seized funds) is directly "traceable" to a "specified unlawful activity". There is a list of those activities in the statutes but again those activities are crimes.
Your analysis would then say if a bank was committing a crime your funds being held at that bank can now be forfeited.
Fun fact about forfeiture is that the government usually does whatever it wants. I know cases where people had their money seized and effectively stolen by the government, even when they had not enough evidence to warrant forfeiture. They simply seized what they could and never gave it back. The money was, depending on the case, locked on the account for years, taken as evidence and never given back or "misplaced". They often take so long to give it back that the owner dies before he sees the money again.
As for the taint, it only works if all exchanges agree not to trade tainted coins, but then they risk that one day all coins will be tainted and they'll have to close business. Most cash in circulation has at some point been used to pay for semi-legal or illegal stuff. Dollar bills contain traces of drugs, blood, semen, you name it. Even a dirty dollar bill is rarely rejected by stores, therefore taint won't work in the long run. It can be a scare for new users, that's it.