Terminal Server Session Management: why do we need a rdp session to take control of a terminal session?

As title, I have this question: why do we need a rdp session to take remote control of a terminal server session?

I copied tsadmin and related file to a windows 8.1 pc and I'm able to see for every terminal server the list of users logged in, but the option "take control" is greyed out... why? is not possible to avoid this block?

Thanks in advance

Davide