PIC18F2550 I2C Open Drain?
The PIC has a built-in peripheral (the MSSP) that allows hardware-based i2c operation.  The open-drain behavior results from the fact that it operates the direction control (aka "TRIS") automatically for these pins.   If the MSSP is disabled, the port pins become ordinary I/O pins and now they're controlled the conventional way, and if programmed as output they will drive high or low.

What was confusing you was the fact that they can operate in either mode.  You'd use the pullup resistors if it were set up for i2c, but not if the port pins were being used as general-purpose I/O.