If trading bitcoin is taxable, then why isn't trading Warcraft gold taxable? It has a USD valuation as well.

I just can't wrap my head on how the IRS diferentiates between bitcoin and warcraft gold(and any other online video game currency). They are both virtual currencies, both can be used to buy virtual and real-life items and services, and both has an arbitrary fiat exchange rate: link via /r/Bitcoin http://bit.ly/2CNUN82