Why opt folder is detained by root by default?

Solution 1:

In linux, the general idea is that all software that is not isolated to a specific user account, should be installed and maintained by an administrator.

Software in /opt can be executed by the user however, so it has 755 permissions by default.

It is important that all executables on the PATH (except those of user scope) are administrator approved, to prevent common pathing exploits. Since software installed in /opt can affect all users, its locked down.

It is also a general rule in linux that all folders in / are owned by root:root. that prevents deletion and renaming. see here for more details and guidelines on the Linux directory structure: http://www.thegeekstuff.com/2010/09/linux-file-system-structure/