Interest

For development : I use symbolic link for all of my plugin then can test on different LimeSurvey version (and all of my repo are oin a specific directory)
If you have 2 instances or more on same server: you can use same files for different instance (you don't do it for limeservices ?)

And there are no reason it was not allowed

Security is certainly a reason.

Security is certainly a reason.

? Why ? It's my choice to get by link and it's only accessible by server admin

Symbolic link work for plugins and for the Survey theme.

It's OK if i fix it ?

You're right, there's no real security reason here. Uploaded ZIPs can't create symlinks, so only the server admin can add one.
Go ahead and fix it. Please follow symlinks only for the theme folders directly in the question theme directory, not in the folders below them, so a symlink loop or a linked repo can't slow down or clutter the theme list.