I know what you're talking about and I understand how frustrating it can be. It seems to me that QB exhibits this behavior when the last Bill transaction you used had an amount in the Expenses tab. It seems to remember which tab was used last. I've tested it and when I enter a Bill with only amounts in the Items tab, it's not necessary for me to go and delete the first line in the Expenses tab. But as soon as I enter one bill using the Expense tab, then the next bill automatically opens on the Expenses tab and I have to delete the first line so it doesn't show up on reports, even though I'm only using the Items tab in that transaction.
It sounds like you have also have Prompt to assign classes turned on under Preferences, Accounting, Company Preferences, Use class tracking for transactions. This would be why you're getting the alert. You can turn off the Prompt to assign classes to get rid of the alert, but the tab behavior will still be the same, in my experience. It remembers the last one you used and opens the next bill on whatever tab was used last.
Someone else may have more insight into this.
