The slippery slope fallacy lies in the assumption that a precedent necessarily sets into motion an chain reaction of inevitable consequences that may not in fact even be related to the initial claim, e.g. "If you let the gays marry, what next? people marrying their pets? It's a slippery slope, man!"

On the other hand, the ratchet effect of political authoritarianism is an observable phenomenon that consistently applies in pretty much every instance known. There is no fallacy in challenging a new expansion of power and asking where it will lead next.