1. They have been lied to about their requirement to pay tax
2. They didn't realise they didn't have to pay taxes, so might feel a sense of power or autonomy and not pay it
3. The legacy of under-funded government budgets has highlighted the irresponsibility of the US Treasury
4. Opposition to public opinion, such as the war in Iraq might incense people to the extent that they no longer want to pay tax
5. Just basic opposition to the idea that the government can expropriate your wealth
Certainly in most countries taxation is illegal. The question to ask yourself - Is it immoral, and what is the relationship between morality and the law.