Since the latest battle.net client update last week, the client behaviour has changed when I switch from wow beta to wow retail. In the past, it would then select my primary, paid, EU wow account. Now, it defaults to the US starter edition I created ages ago for an event, but do not normally use.
I would expect it to either default to the last account I had selected, or failing that to at least prefer a fully-licensed, paid-up account over a starter one.