I think most people don't have accountants. What would happen if you made a gain before you filed your taxes but then shortly after the price crashes. Could you claim that back? And then what about the opposite? You make a loss but then the price rises after you filled. Do you then owe them money?
That should not happen because you are filing for 2014 and accounting for what happened in that year only. You are also only account for bitcoins used to make purchases or traded for other money. So if I bought a BTC for $500 and then "cashed out" by selling it for $300, one could say they suffered a loss of $200. On the other hand if you sold for $600 you owe a percentage of the $100 in capitol gains you have made.
That percentage is highly variable depending on your tax situation. It may be zero or 20%? You have to do your return to find out, but 10% is a reasonable guess. So in the above example you would owe $10 on the $100 you made.