Then why should we give it back to you when you come back? If you could just turn your citizenship on and off with no penalty, it wouldn't be fair to the people who stay in the US if those who leave could just stop paying taxes and then come back whenever they want. And if things don't go well in the US, those who stayed are stuck but those who left have another option. So them not paying taxes to the US would be like having their cake and eating it too. They could leave and stop fulfilling all their obligations, but with the option to come back whenever.