Well, there's also the issue of mission-creep. Microsoft is presumably wanting something narrow enough to avoid scrutiny, but all they need to do is later "discover" that the implementation aspect at stake is in the generic kernel and it becomes about the generic kernel.
Then there's always the risk-o-phobia aspect. If TomTom's implementation is "legally risky", then Microsoft's marketing people need only point out that Red Hat and Ubuntu "could be legally risky" without ever having to attack Linux per-se. Just an implementation. One at a time.
In either case, it's the Roman method of conquest - divide and conquer. So long as they can divide the community into the "targeted" and "everyone else" camps, with the "everyone else" merely spectating, they can rinse and repeat as often as they like. By the time such methods are usually spotted, there's not enough left to form a serious opposition.
Of course, Microsoft might not be wanting to kill Linux by this method, but rather wanting to seize the car navigation market by pwning one of the suppliers and then killing off the rest by sheer bulk, the way they're trying to do with anti-viral stuff. Then we would have to ask whether it makes a difference. Can we afford to ignore anti-competitive behaviour merely because it doesn't hurt us - yet?
As Gandalf noted on his journey to Gondor, it is too late to ask for help when the enemy is upon you.