A、Because Americans used them very often in the seventeenth century. B、Because England wanted to win the naval competition again

Many of the Dutch expressions heard in American English were first used in England in the seventeenth century. (32) That was a time of fierce naval competition between England and the Netherlands. At that time, the British used Dutch as a word for something bad, false or mistaken.
    Some of those old expressions are still used today, with a little different meaning.
    Dutch treat is one example. Long ago, a Dutch treat was a dinner at which the invited guests were expected to pay for their own share of the food and drink. Now, Dutch treat means that when friends go out to have fun, each person pays his own share. Another common expression heard a few years ago was in Dutch. (33) If someone told you that you were in Dutch, they were telling you that you were in trouble.
    Some of the Dutch expressions heard in American English have nothing to do with the Dutch people at all.
    In the seventeen hundreds, Germans who moved to the United States often were called Dutch. (34) This happened because of mistakes in understanding and saying the word Deutsch, the German word for German. Families of these German people still live in the eastern United States, many in the state of Pennsylvania. They are known as the Pennsylvania Dutch.
    President Theodore Roosevelt once noted that anything foreign and non-English was called Dutch. One expression still in use -- to talk to someone like a Dutch uncle -- did come from the Dutch. (35) The Dutch were known for the firm way they raised theft children. So if someone speaks to you like a Dutch uncle, he is speaking in a very severe way. And you should listen to him carefully!