Only the owner of a file and the user root (uid=0) are allowed to change the filemode of a file. It doesn't matter if you have write-access or not, execute-rights or not, etc..

If you have to change the filemode of a file owned by user1 via a command issued by user2 you might want to explore sudo and its possibilities.

Regarding your original question: works as designed.

I hope this helps.

bakunin