BULK INSERT permission on a single database
Ok folks, 

I've asked this on Brazilian msdn forum as well and I got the answer (Forum discussion: BULK INSERT por DATABASE). The user Roberson Ferreira _ helped me. I imagine you can't read Portuguese so I will describe the answer here.

Answer:
It doesn't matter if the ADMINISTER BULK OPERATIONS is server-wide, the user won't be able to BULK INSERT into a certain database if they don't have the INSERT permission as well. The INSERT permission is a database/table-level permission.

Additionally, the ADMINISTER BULK OPERATIONS is not going to grant the user any other permissions over the server or database, all the user will be able to do is BULK INSERT in the database where they have INSERT permission as well.