This is deliberately done so they can change their bonus partners will nilly as they see fit without all the extra hassle and responsibility of having to notify their cardholders all the time. The farthest they will go is to just say they have "bonus partners" that are "major" retailers or hotel groups or airlines or whatever, but you'll never be able to actually nail them down or obtain a list of them. I really don't know why the service providers would be willing to partner with a credit card provider who deliberately doesn't advertise the specific partnership with them. Surely the publicity of the bonus points scheme is all they get out of the deal.
Most purchasing decisions these days are made at home in front of a computer well before the transaction actually takes place, where there is the time and quiet to weigh up all the pros & cons of which vendor to buy from, which payment method to use etc. Part of that decision making process would normally include the prospect of bonus point earns, if you know about them. With Citibank though this is impossible or a guess at best, because they won't tell you specifically who they partner with for bonus points. How do the companies/vendors possibly get any marketing benefit out of such an arrangement?
