This is interesting it's one that I've battled with on my two Windows 8 tablets and an all-in-one PC.
I found a fix for one of my tablets which would be worth a go:
Click on Start ORB
Type settings in the search
Click on Mouse (which will bringup a mouse control panel in desktop mode)
There will be a tab at the top most likely far right which references the touch screen
Under this the touchscreen should be listed and give the ability to edit the settings for that device
You can untick the pinch to zoom here
This appeared on one of my tablets but not on the other, let me know how you get on.