because that's what we were telling our own Congressmen and we've never admitted to having an official agreement with Kuwait that stated that we would assist them if Iraq invaded.
However, Kuwait seemed to think so, as one of their leaders bragged that if Iraq came after them they'd send in the Americans.
Also, Iraq produced after the invasion a document from the CIA that stated (to the effect) that we would offer Kuwait protection from Iraq. (The CIA denies that this is an official document and that no agreement existed - but then again, we run back into the problems with Iraq creating a forgery, if it wanted to create a forgery, wouldn't it have said something far worse?)
And - Bush's final communication to Iraq prior to the invasion did state that the US was concerned - but did not mention Kuwait and did not mention that America would attack.