Now _that's_ a sweeping generalization. Here's a photo from the 2007 Philidelphia Orhestra's program:
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Here's the NY Phil, with a couple standing:
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There are several pics on this page showing the Ravenna festival orchestra, with several standing bassists: Invalid Link Removed
I usually sit when playing orchestra gigs. But I've always seen a mixture, and would be as surprised to see a section all standing or sitting as I would an American section using all German or French bows. Yes, there are some that are totally one way, but I sure wouldn't make a generaliztion about them.