This is one reason why not being anonymous on Steem is not the greatest idea. If Steem grows into a really big thing one day, anyone identifiable on Steem may start getting letters like that. On the other hand, how Steem income is supposed to be taxed is anyone's guess. The tax guidelines where I live are worded in such a way as to allow for the possibility of Steem being treated like a MMOG (Massive Multi-player Online Game), in which case income earned on the platform would only be taxable when it is converted into fiat, cryptos external to it or goods or services. But you never know because those people who have asked about how Steem is supposed to be taxed haven't got any satisfactory answers. Those people who have asked have been taxed only on fiat they have received in exchange for STEEM at least to my knowledge. Another thing is that the platform is constantly evolving and may not meet the criteria for being treated like an MMOG for tax purposes. Because the tax authorities haven't based their answers on their own guidelines, there is a fair bit of uncertainty even now. This is why it is the safest choice to be anonymous.