I think this is actually a really interesting parallel. They're both virtual currencies, but one carries actual legal risk and the other doesn't. The question is, are they really all that different?
I think it's mostly a matter of FinCEN et. al. not yet training their sights on ISK, because ISK is not widely supported in terms of fiat-exchange and online purchases.
There's certainly no reason someone couldn't accept ISK on Silk Road in exchange for their magic mushrooms, but the reality is, why take ISK when you could take Bitcoin which has better tools/software for trading, better liquidity, and no centralized control?