I agree with you, USB cables like HDMI cables, are transmitting a digital signal, and provided the integrity of the cable is fine, then one cable should do exactly the same as any other. However, it hasn't stopped the spawning of a minor industry in selling a wide range of USB cables, at prices in to the $'000's.
Check out the blog post 'Archimago's Musings' ( http://archimago.blogspot.com.au/ ) for a series of objective, and subjective reviews of this sort of stuff. If you go back through the last few months of articles there's some good stuff in there. ( I loved his review of power chords, where amongst others, he compared a 25ft length of old outdoor cable - the cable that he used, literally to feed his Christmas lights over the last few years).
I respect those who do find benefit in a 'good' USB cable, and I do not challenge them, nor ridicule them. If that choice is based on an informed decision even better, and if someone can afford it, then a $50 USB cable, or even a $200 USB cable might buy peace of mind, if not that last piece in the jigsaw that they're looking for. I've bought a couple of $60 ish USB cables - peace of mind, rather than a 'sparkling top end' or whatever, if truth be told. I'm comfortable with that.
Overwhelmingly, and by an enormous margin, the quality of what I listen to is affected by the source, i.e. the recording I am listening too. I have around 100 music cassette tapes I have recorded into 16/44 FLAC - no USB cable will give me what doesn't exist in those recordings (and they're crap for the most part). With 24/96 recordings, I've yet to discern any difference using any reasonable quality USB cable.