The problem is that slippery slopes are often real, and citing it as a fallacy is normally done to dismiss the idea that it could be real, without making an argument. As you say, whether one thing will lead to another depends on circumstances. But a fallacy is supposed to be an argument that is wrong because of faulty logic. A claim that one thing will lead to another can be wrong, but I would say that it's almost always wrong because the underlying premise is wrong, not because there is a claim of an existence of a slippery slope. For example the "gay marriage -> child abuse" rhetoric is coming from religious conservatives who likely believe that strict adherence to their religious rules and practice is the main thing keeping society from "degeneracy" and general bad behavior. Given the premise, the conclusion isn't illogical, the problem is that the premise is wrong. Instead of calling it a fallacy, it would be a better argument to have the premise clarified, and make an argument against its merits.

In the case of the OP situation, I would say that when a company is actively using tools to examine and control the contents of a user's device, that makes it more plausible for demands that they expand what they do this for will be followed. I'm sure plenty of people would try to dismiss that as a fallacy, but really it's a claim about how things work.