It happens because %post section is chrooted to /mnt/sysimage (where the system you are installing is).
To get access to /tmp created in your %pre, you need to use "--nochroot " option in the %post section. This may lead to create 2 distinct post sections (one with "--nochroot " the other without), or you'll need to prefix any path with /mnt/sysimage.
take a look at redhat doc: "https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Installation_Guide/s1-kickstart2-postinstallconfig.html"