How to add backports to preseed, no pinning by default

Hi,

When I do Wheezy preseed installation with:
d-i apt-setup/local2/repository string http://caroline/debian wheezy-backports main contrib non-free
d-i apt-setup/local2/comment string Debian Backports repository

installation goes through ok, but after boot I DONT HAVE network available.
Something has been installed from the backports repository over
normal wheezy repository and it's messing up the network stuff.

If I comment out those 2 lines in the preseed, network will be just fine.

Also if I do normal wheezy installation from USB-stick, reboot and then do:
echo "deb http://caroline/debian wheezy-backports main contrib non-free" >> /etc/apt/sources.list
apt-get update
apt-get install pepperflashplugin-nonfree
it will install pepperflashplugin ok.
should I not always tell which repository use unless is the main one:
apt-get install -t wheezy-backports pepperflashplugin-nonfree
or is it so, that when package is found only in one repo, that will be used?

Also there is no pinning set by default though backports instructions say:
http://backports.debian.org/Instructions/
"All backports are deactivated by default (i.e. the packages are pinned to 100
by using ButAutomaticUpgrades: yes in the Release files."
does not look like it since preseed does take stuff from there just fine...

There is no /etc/apt/preferences.
Only /etc/apt/preferences.d/ which is empty.

Any ideas, is this a bug or a feature (which I did not understand correctly)?
And if it is a feature, how can I add backports to the preseed so that it will
not take stuff from there automatically, only when I tell by using
-t wheezy-backports?

Thanks.

ps. in the preseed lines "caroline" is my home network reprepro server :)