There are many countries that don't require a pledge of allegiance for citizenship, so you can't really argue that being a citizen of those countries constitute

There are many countries that don't require a pledge of allegiance for citizenship, so you can't really argue that being a citizen of those countries constitutes a conflict of interest with US citizenship. Being a citizen of a country typically means that you have certain rights and privileges in that country that non-citizens don't. That's about it. "Loyalty" doesn't enter the picture unless one is a hardcore nationalist.