So declaring bitcoins illegal will end bitcoins as we know it today.
How do you declare bitcoins illegal ?
As long as I can send a signed (transaction) message or a private key to somenone, bitcoins can be traded, right ?
A business accepting bitcoins can display prices in fiat as long as bitcoins are traded and they can make a reasonable assumption about a conversion rate.
The shoppers are not obliged to pay in bitcoins anyway.
The US government could just declare usage of bitcoin illegal and no US shop will accept them anymore.
Likewise for any other country.