The way the law works in the States, you're expected to defend every trademark infringement. If it can be shown that you let one slide, it can be taken as a sign that you'd don't enforce your own copyright.

And certainly, in a case where it is SO EASY TO CONFUSE a pick up truck with an F1 racer, you can see how important this is. 


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"We all love the music. What we hope is that the music loves us." -- Tom Waits