It does not make sense to say Bitcoin is "legal" or "illegal". Bitcoin is a software program and cannot be arrested. Certiain actions by people in certain places may be illegal.
Your argument is specious at best. One could apply the same logic to possessing child pornography by saying "They're just computer files with 1's and 0's in them so how can they be legal or illegal?"
Governments can declare anything they want legal or illegal through whatever legislative process they have. Could the declaration be silly and unenforceable? Of course. That being said, the govt could still throw a big damper on BTC and drive it back underground where it started (Silk Road, etc)
The easiest government attack on BTC is to ban exchanging it for fiat currencies like USD. Another easily enforced attack would be to declare it illegal for use as payment by stores. Imagine a time in a few years where BTC has become more widely accepted to the point where Amazon accepts it. This situation might cause concern for the US govt. If mainstream online and brick-and-mortar stores were to be banned from accepting BTC (easily enforceable) it would effectively shut down BTC.
Activities like mining BTC, possessing BTC, exchanging BTC for goods and services
between individuals couldn't really be banned at least not enforceably.
But nonetheless BTC would be driven back into the shadows.