Unlike income, streams of revenues that are particularly categorized under the laws are taxable, cryptocurrencies have a league on their own in which a law must be passed in order for them to be taxable. Currently, most countries are silent on the status of BTC in their country (e.g. they chose to tolerate it, etc.). But once there is no law declaring it to be taxable, then I guess it is unfair to say that BTC allows tax evaders.
If I get in, and never get out then I am never taxed, because it is unrealized at that moment. This is the most perfect way and we are very happy about it. Since I earn mine and not "profit" from trading, mine is charged like income which is not really that well and I am paying higher amount but it is fine so far, as long as it is legal and regulated.