Sometimes expiration dates refer to when enough plastic from the packaging has decayed into the food material that it might be a problem. Bottled water works that way.
I don't know:
- How much science there is behind the dating
- How much plastic you're consuming in your food anyway and so who cares what's the difference
- Whether that's what's going on with this salt package specifically
But it's not automatically crazy for there to be an expiration date on an immortal product if it comes packaged up in plastic.