we do not have any tax on us for having bitcoins but while buying bitcoins we do pay a fess and maybe tax to those companies which are involve in that purchase and that companies do their work to collect the tax so you do not worry for that they will get the tax for their respective country.
yes that is a fact that we do not have tax on bitcoin but the only reason is that bitcoin is still not consider as legal currency by any government , therefore it in most of the countries people are using bitcoin illegally. but as soon as government will consider bitcoin as legal l currency then government must put tax on gambling.