Featured artists usually do get credit:
You can blame only Interscope for Britney not getting credited, since the following info is submitted by the label upon certification request:
- Music Company
- Music Label
- Artist(s)
- Title
- Configuration (single, shortform, album or music video)
- Level (Gold®, Platinum® and/or Multi-Platinum™)
- Selection or Catalog Number
- Release Date
- Billing Contact & Address
Her label has nothing to do with it since it was released under Interscope. Pretty pathetic of them to not credit her.