The problem is that, even if you think this is the kind of thing the government should be able to do, they haven't told me which ones are "radioactive". So if someone uses one of those bitcoins to buy something from me, I might have trouble spending it afterwards. Failures of fungibility like that are potentially a big problem for a currency. So far they're only a small problem for Bitcoin, but as these examples show, they have the potential to become significant. That might create pressure to move to a coin like ZCash, Monero, or Mimblewimble which has built-in safeguards against such things.
