Hello @Thoast
Welcome to the forum, and thank you for the question!
The reason for this limitation is because of the VNC protocol, which ThinLinc in based upon, only supports a limited number of buttons. An extension to the protocol would need to be developed for this functionality.
This is known to us, and is being tracking in our Bugzilla at
https://bugzilla.cendio.com/show_bug.cgi?id=7459