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.
No doctor is forced to operate his patients. A doctor might be forced to inform his patients that it's possible cure his illness by surgery.
There no need to argue for slippery slope, it's quite possible to write laws that allow doctors euthanasia but make clear that they are not forced to do so. Additionally you can also put other restriction on it like marketing bans.
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.