mattiastofte:
. . . I dont understand why this is. . .
Here's an over-simplified explanation:
When at rest the signal lines (SIOC and SIOD) are supposed to be at a (relatively) high level.
Communication is established when a device periodically brings the signal levels to a (relatively) low level.
So there is a power supply of some sort trying to make the signal levels high and there are devices that are periodically trying to make the signal levels low. If you do both at the same time you have what is commonly called a 'short-circuit'.
The pull-up resistors prevent this short-circuit condition. Their value must be low enough so that the high signal level is correct and high enough so the current isn't excessive when the low signal level is present.
Did you ever wonder why it takes so many years to get a degree in engineering?
Don