Tiny trace fusing and blowing the 16u2

Does anyone know the reason why the Mega (unsure about the Uno) has a miniscule “jumper-trace” on the USB ground line near the USB socket? I’ve had this fuse twice and blow up the 16u2 since it’s then providing the only ground path through itself. I’ve bridged it on the replacement board, but I’d like to know its intended purpose. It certainly can’t carry 500mA! Attached is a screenie of the jumper-trace from the board layout, it’s right in the centre of the picture.

According to an online calculator, it ought to be sized OK for 500mA, even if it's 0.5oz copper. While only 12mil wide, it's also only about 25mil long, and predicted temperature rise is only about 30C. Certainly far short of fusing...

I don't know why it is there. (And it is present on Uno as well.)

Is it so you can isolate ground from the USB socket metal casing?

If you're using a separate power supply with the USB connected it would prevent the ground current from both wrecking your computer.