I'm not sure if I buy this argument; the legislature need not, and generally does not, work on the basis of previous precedent. There are two main responses to the idea that we ought not to do X because it's a slippery slope to Y in political theory: firstly, people disagree about how bad Y is (unless Y is made patently absurd); secondly, slippery slopes require justification.

It's evident that one can't simply allege the existence of any slippery slope - for example, the 'slippery slope' of legal gay marriage leading to adult-child and human-animal relationships. So there must be some work put in to justify the existence of a slope which is claimed to exist.

In particular as it relates to issues of free speech and 'facial recognition', I have seen no evidence presented to justify this slope as a rhetorical or political device.