Since currency can be owned, it is someone's property legally
The difference is that property is usually taxed (a property tax) while currency itself is not taxable overall. There were efforts which could be construed as directed at taxing the money you own (for example, free money), but as far as I know, this idea didn't catch on anywhere. Really, why would you have to pay a tax if you decided to save some funds to buy, say, a house? The bottom line is that money is not taxable, but property is, so if Bitcoin is to be considered as property, it should be taxed accordingly