Legalizing euthanasia doesn't force any doctor to engage in it. It just removes punishment when a doctor engages in it.
In theory you are correct. In practice, the way similar issues have played out in the US is that regulators start applying reasoning like Jiro exhibited elsewhere in this thread.
Over at Scott Adams' Blog you can find a very fine example of using the 'Rationality Engine' to solve the social problem of assisted dying.