I missed the link for unsubscribing from one single product.
You are right, it does not make any sense at all.
The only difference I found is that, when an e-mail is sent with a stock alert, it is marked as
sent in the database.
When you click to unsubscribe from one product stock alert, the record is deleted from the database.
But the effect is the same. You never get the same e-mail again unless you subscribe again for the stock alert and the product goes back in stock the second time.
Here is a scenario where this is valid.
You subscribe to the stock alert for 2 products. Let's call them A & B.
Then one night A goes back to stock.
You receive the e-mail that A is back in stock.
You click on the link
Unsubscribe from all stock alerts.
You are unsubscribed from ALL stock alerts.
Next night, B comes back in stock.
You are NOT notified about this because you unsubscribed.