Maybe the government has fear on it, because we all know how facebook itself dominates in the world in terms of business, then now they will be introducing a new stablecoin called libra, it could be possible and might be its a threat for US financial system, im not really sure, because libra is not yet out in the market this time.
Mark is a smart guy and intelligent to create a system that will totally affect or disregard the system of the government, he would not want to have any challenge too with the government and I believe that any system he is creating may likely even benefit the government more.
Government will surely be threatened after knowing what the blockchain technology is all about, and the fear or hatred they have for one of the products that was produced out of it, they are yet to see the negative impact of cryptocurrency on them and they are watching slowly to see how it gradually gets adopted, and what will be the outcome of its adoption.
With Facebook adoption, it means there will be sharp adoption which will override the way they gradually monitor the adoption for them to know if the use of cryptocurrency will have negative impact, but I am sure that Facebook will end up helping them in creating the centralized system they wished for.