Not really an Oracle Text question, but I think I can answer it anyway.
When you do the install, it asks you for the operating system group for the Oracle owner. This is usually known as the "dba" group, and traditionally it was fixed to a group actually called "dba", but these days can be any group.
Any user who is in this group can logon as SYS without specifying a password - and if a password is specified, it can be anything.
That's the situation on Unix/Linux, anyway - on Windows I suspect it's just the user who installed the software who has these rights.
Not sure it's possible to change the dba group after installation, but a Google search for "oracle change dba group" turns up a promising list of results.