Governments, like the US government, have several reasons to buy bitcoin.
Countries that are early adopters of Crypto-currencies will have a huge advantage in the future. I believe some governments have realized this and have been buying.
Government wants and needs control in order to stay in power, and prevent others from taking over. If they buy a lot of Crypto-currencies they can control the price fluctuations(if they buy enough).
Buying bitcoin and other alts would be a great way for governments like US and China(esp if working together) to further inflate the dollar, by driving the price up and rewarding early adopters.
By buying a lot and then selling it all, in conjunction with negative reports and negative news, they could most likely kill bitcoin and alts if they wanted to, in order to keep banks in control.