Question: I know for the cheaper items that apple doesn't bother. But given how expensive the magic trackpad is, it seems foolish to not include it in the online warranty database.
ragingtonberry answers: It's because they aren't an activated product.
When Apple ships a Mac/iPhone/iPad to a customer or an Apple Store or a retailer, the manifest has every SKU and serial number on it so each individual unit can be tracked and located. When a sale is made, the exact SKU and serial number is scanned in at the point of sale and is "activated" in Apple's systems with its date of sale. This also sets the warranty period depending on what country you're in (for the US, it's one year).
For non-activated products like keyboards, mice, cases, cables, etc. they aren't tracked from factory to POS even if they do have serial numbers. Magic Mouse shipments come in boxes with multiple units per box just like iPads, but a major difference is the iPad box has an additional label on it listing all the serials contained within the shipment. So at each checkpoint, not only is the box scanned in, but each serial as well.
So a box of 12 Magic Mouse 2s is only tracked by SKU and count from factory to customer and the serial is mainly used to track service and repairs.