I have noticed this too; the standing ovation is the rule in the U.S. (at least in places where I have been). In a way, I am surprised at this. In their daily lives, people tend to be rather unfriendly and rude (at least where I live and many of the places in the U.S. I have visited); it is funny that they suddenly become overly nice in the concert hall. Given the general level of incivility in the culture, I might expect the opposite.
However, there are noticeable differences. Even though just about every concert gets a standing ovation at the end, the level of enthusiasm varies considerably. For example, an "average" Utah Symphony concert gets polite applause and a gradual standing ovation, with maybe two curtain calls. But sometimes when a performance really resonates with the audience, such as the performance of Mahler's Seventh a few weeks ago, the crowd's enthusiasm is palpable; there are more shouts of "Bravo", and the crowd jumps to its feet immediately. I also saw this after an especially moving and powerful performance (by a student) of the final scene from Suor Angelica at the gala concert of the University of Utah School of Music. So one can tell the difference between the obligatory (and somewhat reluctant) standing ovation and the real thing. It's obvious, and I am sure the musicians understand it well.
Heather