Thanks for your answer. Just wanted to PM you. If you read the leading comment in uc_stock.module you find:
... Store admins can set
* the stock levels on a product edit page and a threshold for each SKU value
* When that threshold is reached admins can be optionally notified about the
* current stock level...
So I am sure it is meant like this: We sell until stock is zero, but the admin can get a notification at threshold.