As a concept, slippery slope isn't well understood. On the face of it a lot of these kinds of arguments can be very specious and absurd. For example, if we allow homosexuals to marry it won't be far off before we can marry our pets. Or, if we socialize medicine, why not haircuts while we're at it?
That kind of thoughtless extrapolation will bulldoze right through natural stopping points on the slope. Obviously marriage should be for people, and bad hair days aren't a matter of life and death, right? So it's easily apparent even to the least informed how irrational those formulations of the argument are.
If you unpack the rationale behind slippery slope though, it's really not that insane. The concern is always that some principle is being altered, or degraded specifically. Some people see regress where others see progress.
The seatbelt law "slippery slope" isn't concerned just with seatbelts. Unpacking it just a little, the principle that appears under "attack" is freedom. Freedom vs. safety (and what a beloved debate this gem is), and the question as always is "should the state be nannying us?"
This should be fairly easy to dismiss out of hand. What freedom is it you're specifically concerned about? The freedom to die horrifically and preventably in a car wreck? Anyone who's life was saved by a seat belt they were forced to wear should have the good grace to thank you afterward. To be sure, I have no doubt that seatbelts save lives and the statistics reflect that. Everybody should wear a seatbelt.
BUT, this is missing the point. Yes, there are always concerns about freedom when a new law is passed (which are usually ignored since preserving the freedom to do something objectively stupid doesn't get a lot of people hopping), but that's really NOT the primary concern. The actual principle is individual responsibility. The actual question is, "what truly makes us safe?"
The thinking is that knowledge makes us safer. Conscientiousness makes us safer. Understanding risk makes us safer. Knowing that even if you do everything right you can still have an accident, will make you safer. And if you're this kind of person, you don't need to be told to wear a seatbelt.
Compulsion can make you safer. The threat of a ticket can make you safer. But that benefit stops when you step outside your car. What else are you going to do today? Drive a forklift? Smoke a cigarette? Cross the street at rush hour? Shingle