I did some more testing and was able to narrow down the problem area.
Looks like my repository is not needed at all to reproduce this.
I made 2 installations (no GUI):
first one does not have the backports in the preseed at all --> network ok (got "apt-cache policy" and "dpkg -l" outputs to files).
second one does have backports in the preseed --> network NOT ok (same info to files too).
here is policy from the one with backports added in the preseed file:
Package files:
100 /var/lib/dpkg/status
release a=now
500
http://ftp.fi.debian.org/debian/ wheezy-updates/main Translation-en
500
http://ftp.fi.debian.org/debian/ wheezy-updates/main i386 Packages
release o=Debian,a=stable-updates,n=wheezy-updates,l=Debian,c=main
origin
ftp.fi.debian.org
500
http://security.debian.org/ wheezy/updates/main Translation-en
500
http://security.debian.org/ wheezy/updates/main i386 Packages
release v=7.0,o=Debian,a=stable,n=wheezy,l=Debian-Security,c=main
origin
security.debian.org 500
http://ftp.fi.debian.org/debian/ wheezy-backports/main Translation-en
100
http://ftp.fi.debian.org/debian/ wheezy-backports/main i386 Packages
release v=,o=Debian Backports,a=wheezy-backports,n=wheezy-backports,l=Debian Backports,c=main
origin
ftp.fi.debian.org 500
http://ftp.fi.debian.org/debian/ wheezy/main Translation-en
500
http://ftp.fi.debian.org/debian/ wheezy/main i386 Packages
release v=7.6,o=Debian,a=stable,n=wheezy,l=Debian,c=main
origin
ftp.fi.debian.orgPinned packages:
So 100 is the value for backports packages, ok.
But why translations are 500 making it same level as in main?
Anyway, things got interesting when I did diff with the "dpkg -l" output files:
76c76
< ii ifupdown 0.7.8 i386 high level tools to configure network interfaces
---
> rc ifupdown 0.7.8 i386 high level tools to configure network interfaces
83c83,84
< ii iproute 20120521-3+b3 i386 networking and traffic control tools
---
> rc iproute 20120521-3+b3 i386 networking and traffic control tools
> ii iproute2 3.15.0-2~bpo70+1 i386 networking and traffic control tools
86c87
< ii isc-dhcp-client 4.2.2.dfsg.1-5+deb70u6 i386 ISC DHCP client
---
> rc isc-dhcp-client 4.2.2.dfsg.1-5+deb70u6 i386 ISC DHCP client
100a102
> ii libatm1:i386 1:2.5.1-1.5 i386 shared library for ATM (Asynchronous Transfer Mode)
Why is iproute2 installed from backports?
Guess that is our problem here.