Parliamentary democracy works on the basis that voters choose the type of government they want through proxies.
Specifically, a conservative voter might like Stephen Harper as Prime Minister but that voter cannot (unless they live in Mr Harper's riding) vote for him - the voter has to vote for someone else who supports Mr Harper. Similarly someone who likes Mr Harper's policies on, say, crime cannot vote directly on the issue but must vote for a member of Mr. Harper's party in the expectation that member will vote accordingly.
That means, of course, that some of the freedom an individual MP might have has to be limited so as the ensure that, broadly put, all Conservatives support the same policies.
And that's true for Conservatives, Liberals or members of the NDP.
As a result from time to time a leader (of any party) has to say "this is what we believe" and hold anyone running for that party to those beliefs. This isn't a matter of imposing policy - it's a matter of allowing voters to make choices.
Parliamentary democracy requires political parties to have a unified core of policies - without them the system fails.