On the surface this appears to be very short-sighted on Ford's part.
You have to dig deeper to try to figure this out. $10K dollars? That would pay the law firm for ONE DAY, if that. Maybe less than a day. So they're obviously not in this for the money.
The aftermarket, magazine, and internet community serve to enhance the experience for Mustang faithful. To go after these folks seems short-sighted in the extreme. The only benefit here might be if Ford wanted to enter this market themselves. They don't. They don't have the breadth,... it's not a great value for them (vs. staying with what they do, build cars).
Having seen similar situations first hand, the only thing I can think of is that if companies don't "defend" their trademarks, then others can just take them. That is, maybe someday some Chinese company will make a car and start selling it as a "Mustang" (just a silly example to make the point..). I'm not defending this notion, just pointing out what Ford might be thinking.
The only other thing I can think of is... in cases like this, lawyers win and everyone else looses. The law firm has worked up someone within Ford to get them to think that this is something they have to do... etc, etc...