It's taxable as property, not currency. they want the owner of the property to translate the value of the property at the time it was received. So there are loopholes already within their assessment virtual currency. If they were to attempt to tax it as currency, that would open up a whole big can of worms.
Even this can be argued that it isn't "property" because it isn't tangible. There are no laws "yet" that prove it is a "thing". I have been saying that is its only intrinsic value. Once it becomes a recognized form of currency, and or gets publicly traded under the SEC (like an ETF) that will be all for virtual currencies.