why can I write to a file I don't own??

[David.Knight at clubcorp.com]

RedHat List,
        I was working on a script the other day and ran into an anomaly 
with the file permission's on files. I have checked this on several ES 
servers and all produce the same results. Say a file has the following 
perms: 644  and it is owner and group are root:root. as long as a user has 
write permission's to the directory it is in they can write to it. not 
only that the UID:GID change to that user. I am running ext3 file systems 
with kernel 2.4.21-20.ELsmp. So my question is 

1) why is this allowed?
2) can I change this?

# pwd
/home/test_dir
# rm test.fil
# pwd
/home/test_dir
# ls -ld .
drwxr-xr-x    2 user7  root         4096 Apr 14 16:56 .
# id
uid=0(root) gid=0(root) 
groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)
# echo "test from root" > test.fil
# ls -l test.fil
-rw-r--r--    1 root     root           15 Apr 14 16:57 test.fil
# su - user7
$vi test.fil
$ ls -l test.fil
-rw-r--r--    1 user7  user7        31 Apr 14 16:57 test.fil
$ cat test.fil
test from root
test from uset7

However it doesn't let you echo "test from user7" > ./test.fil. it 
responds correctly......
Any thoughts on this would be great.
-David Knight