tedster - 9:41 pm on Aug 27, 2010 (gmt 0)

why artificially limit the audience the site is shown to?

Only if you can't serve the international audience (or choose not to). Some offerings are like that.

can Google, once learned that the webmaster wants to target the site to US visitors, forget about it after I change GWT settings?

Theoretically, yes. I've only heard one practical report about this and the recovery took many months for them, but they say it did happen eventually.