I'm really surprised so many people find it rude. I mean, you use a restroom for a certain reason, and someone else talking on their phone doesn't interfere with taking care of your 'business'. I'm just imagining saying to someone - "I'm sorry but can you take your call somewhere else? I'm trying to pee, and you're bothering me." It makes me laugh.
Not that anyone would actually say that, but the sentiment is there. I figure it's a public place, and if someone doesn't care if other people can hear their conversation, well, it's no biggie to me. Now, I
don't really want other people to hear my private conversations, but not everyone is like me - and that's okay!