I know it's only semantics, but we're talking about 2 different things here.
Tips are now added to your onboard account automatically. They are actually charged to your CC (or however you are paying your onboard account) at the end of the cruise.
"Pre-paid tips" are actually paid ahead as part of the cruise fare that you pay. That way the tips are already paid for and you don't have to budget for them to be paid at the end of the cruise.
I will also point out that you can go to Guest Services once onboard and remove/change your tips as you deem necessary.