Basically the manufacturer cannot control how much you load the product once you have it they don't want you to put anything on the rack at all as it might change the way the bike feels causing you to have difficulties hence the load warning

So if your told that the manufacturer rates an item at 5lbs load limit and you put on 10lbs then you absolve them of liability.

The fact that the item in reality can hold 100lbs and that a similar model vehicle already has more weight in that location is your choice but is against the manufactureres recommendation hence on your head.

As with all things commonsense must prevail