The government has the power to tax, that we must remember at all times. In many countries, not declaring and paying correct amount of tax has big penalties attached into it so people are always making sure that they are not violating the law otherwise it can be a big hell to thresh out with once you are entangled with the government authorities.
Yes, I agree because the government has the power of taxation. This what they'll most likely do, impose taxes on Bitcoin if a country's government were to regulate it. Imposing taxes allows the government to generate revenue, and the government will surely use it up to their advantage.
Tax exemption is punishable by law and one thing they can do to avoid this is to come up with some sort of tracking system for bitcoin transactions so that no one may get away with it. If the government were to regulate Bitcoin, I'm pretty sure that they'll come up with various rules and regulations just to gain control.