While trying to do `su root` from normal user, why does Red Hat Enterprise Linux 5.8 not prompt for password?

Solution Verified - Updated 2013-01-25T08:30:04+00:00 -

Issue

Whenever any normal user it trying to do su root is getting successfully switched to root user, but its not asking for root password.
Is there any particular reason for the same?

Red Hat Enterprise Linux

5.8

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