[Q] Rooted Samsung can not remove apk file in /system/app

My Samsung Galaxy have been inject some virus in /system/app. Now I root my phone and run rm command by root user, but still can not remove the apk file. The system show:

rm failed for /system/app/providerdown.apk, Operation not permitted

I want to know why the root use don't have the permission to remove some file?

I post the cmd log of my operation:

root@android:/system/app # ll -la |grep providerdown.apk

-rwsr-sr-x root root 178557 2015-06-11 14:07 providerdown.apk

root@android:/system/app # mount -o remount,rw /system

root@android:/system/app # rm -rf /system/app/providerdown.apk

rm failed for /system/app/providerdown.apk, Operation not permitted

The system file system have already mounted, and I can touch a new file, and also can remove new file by rm command.
Can Selinux affect the root permission, or there have some other permission that upon root user?

Any one can help me and Thank you for your ANSWER!