1 - Preventing trademark erosion. Companies have a duty to be proactive and continually challenge trademark infringements. If not, the trademark can become invalid. e.g. this happened with Sony's Walkman (TM).
However, they don't seem to have good grounds for it in this case. They are dissimilar enough (both in visual style and domain) that no reasonable consumer would get confused.
2 - Over-eagerness from the legal department. [Speculation, may not apply to Apple.] Pressure to get billables, to prove individual relevance, or even just busywork can -- in the absence of clear guidelines and policy -- lead to the legal dept taking unnecessary action. This becomes more true the more silo'd the different departments in a company are.
I believe this is a natural thing for big companies to evolve into, which can be addressed amongst other things through SOP's, guidelines, oversight (i.e. more people signing off).
Suffice to say that your average conglomerate cease and desist has not passed the desk of the CEO.