SSH is a secure form of Telnet connection, using an RSA key. It is a more powerful means of access than FTP, as it provides a command line interface for running scripts, etc.
At the same time, SSH allows more possibilities for error, and actions which may cause harm to the server. Therefore, SSH access is a privilege to an account. It can be revoked at any time if user's activities appear detrimental to the health of the server.