In President Kennedy's quote, the tiger represents the dangerous and powerful forces that some people try to control or manipulate in order to gain power. By saying that those who rode the back of the tiger ended up inside, he means that those who try to exploit or control such forces often end up being consumed or harmed by them.
In relation to supporting freedom, this quote serves as a warning against using oppressive or authoritarian means to achieve power. It emphasizes the importance of upholding and protecting freedom by avoiding tactics that may ultimately undermine it.