Nothing protects you from a government that is willing to imprison and torture you and your family, friends and associates. But in Silk Roads case, they had a murder for hire plot, knew the defendants had bitcoin, and the defendants were willing to bargain to reduce sentences.
If you are a businessman in Venezuela, it's unlikely the local corrupt politicians will know you have bitcoin. They can search your properties and find cash, bullion, etc, but they should never be able to find your bitcoin. If you travel or leave the country you dont need to hide it on your person to get away.
Once out of the country you can easily turn it into cash and other assets. You can easily return and rinse/repeat.
Yes - you are right, right up to the point where BTC becomes a tax collection issue on the radar of the US authorities. At which case turning it into other assets become probably less easy (because it hasn't got a consumer utility) than converting hard drugs to untraceable cash.
On that note, cash is also impossible to ban.