When someone sets a target ACOS at the campaign level -> all the keywords in there inherit the Target ACOS.
If a user then goes in and ADDS ANOTHER keyword to that ad group, they should set a target ACOS for that keyword.
We're planning to introduce a feature where keywords will inherit the campaign level setting by default when users add keywords or graduate items. By default, we haven't implemented this because our users have the flexibility to set individual target ACOS and minimum/maximum bids for each keyword. However, to simplify the process for our users who prefer not to customize, we'll soon be implementing a system where keywords will automatically use the campaign settings.