strange newfs results

hi there,

i was surprised to see the following:

1. pax -w -f /dev/ad0p2 .
2. file -s /dev/ad0p2 -> POSIX tar archive
3. newfs -U -L usr /dev/ad0p2
4. file -s /dev/ad0p2 -> POSIX tar archive

mounting/reading/writing ad0p2 however works like one expects from a ufs
partition. i added a `dd if=/dev/zero of=/dev/ad0p2 bs=1m count=10` between
step 2 and 3 and now ad0p2 is recognised by `file -s` as "Unix Fast File
system".

is this a bug or a feature? i'm running r193846M.

cheers.