Yes when u say stealing then yes its illegal.And bitcoin is a currency its also a money so stealing it surely illegal but what you are talking about is dont think gonna happen.The chance to that happen is very very less.
Bitcoin is money but not a legal money so that is not illegal, I hope you know guys what are the difference of not right and illegal. Maybe it is not right to steal but you cannot say that it is illegal because bitcoin is an independent currency and the government does not recognize it.
If the authorities indeed does not recognize bitcoin and cryptocurrencies in general as you say, those dealing with them would not have been walking the streets free. Fact is the authorities are still fine tuning their position on this new technology and that doesn't make it illegal like you would say for things like hard drugs.
At any rate, taking whatsoever that does not belong to you, even if it be garbage, without the permission of the owner is stealing no matter what color you chose to paint it. Bottomline is to do to others, what you want others to do to you.