As I mentioned, on a real MCU the data sheet is silent on what value (if any) is in the SSPBUF if you don't write to it. I have even less idea what your simulator is doing but, going by an apparent interpretation of the trace you have shown, it is putting in zeros.
The way I read that trace, you are looking at both the DIN and DOUT lines of the SPI device at the bottom left of the image. Because the SPI protocol is always an exchange of values and you are not writing to the SSPBUF when a particular MCU is playing the 'slave' role, you are seeing the values form the 'master' and zeros form the 'slave', which is exactly what your code says should happen.
Susan