Treaties are no more sure to be right than statutes. A democracy needs to have laws that suit the people, so being able to get out of treaties is what the Founders had in mind when they put treaties on a par with statutes. http://en.wikipedia.org/wiki/Article_Six_of_the_United_States_Constitution
But treaties are confusing — a thicket. Some countries internal laws do make treaties superior, so their hands are tied, while ours aren’t. The Netherlands, France, Switzerland, Japan, and Belgium are stuck in that way. http://www.asil.org/ajil/v86310.pdf, p.320.
[UPDATE April 2016: That was a bad link. Here is a better one:
There is also significant variation among monist states concerning the hierarchical rank of treaties within the domestic legal order. In Austria, Egypt, Germany, and the United States, treaties are equivalent to statutes; they rank lower than the Constitution.79 In South Africa, treaties rank lower than statutes.80 In China, France, Japan, Mexico, and Poland, (at least some) treaties rank higher than statutes but lower than the Constitution.81 In the Netherlands, some treaties rank higher than the Constitution.82 In Chile, Russia and Switzerland, the hierarchical rank of treaties is contested, but it is undisputed that at least some treaties rank higher than statutes,83 and there is some authority for the proposition that some treaties have constitutional rank.84 http://digitalcommons.law.scu.edu/cgi/viewcontent.cgi?article=1620&context=facpubs.]
So it irritates them when we use the Founders’ rule. Understandably.
Here is an article looking way back to the Founders, and pointing out that in recent years, “the United States has drawn international criticism for overriding bilateral tax treaty obligations through changes to its tax laws.”
So as not to rub it in, Congress uses the word “override” when the time comes. Not violate, not breach, not abrogate. See Tax Treaty Overrides: A Qualified Defense of US Practice.