That's not entirely true. The law can make crypto currencies illegal and put them outside the banking system. The US government did this with online gambling payment systems like NetellerC which severely crippled that industry. They also could potentially charge the founding developers and ICOs. Basically if the law makes it too difficult and non-convertible to real currencies and mainstream goods and services then they can exert the necessary influence that will either kill it or force change via the major actors.