The slippery slope argument is not always a fallacy. The strength of a slippery slope argument relies on the ability to show that the initial action will actually lead to the predicted outcome. The fallacy comes in when connections are drawn between unrelated concepts - an easy example of this is the argument that legalizing abortion will lead to the legalization of murder. In this case, I think it's pretty likely that making a certain item legal to steal will pave the way for more items to be legal to steal in the future. After all, who decides which items should fall under that law? I'm sure there will be plenty of people with very strong, differing opinions on the topic.

Because this tends to happen in law, especially when it sets a precedent for future cases.

Would you care to provide some additional context? On some levels I agree with you, but I would be interested in hearing the rest of your thoughts on the matter.