Trademark is exactly supposed to protect the reputation of WordPress. If people confuse WPEngine for WordPress, and assume if WPEngine doesn't have it, WordPress doesn't then that's damaging to WordPress. Except that WordPress doesn't (and legally can't) claim "WP". I'd be frustrated.
This is like asking what is it a concern of McDonald's if you open up your own restaurant call McDonald in your town and make it a dump
I don't know about three letters but two letters cannot be trademarked. The exact point is that WordPress cannot do anything legally, and that's understandably frustrating.
Doesn't this serve as evidence that my speculation that Matt is frustrated with the behavior of WP engine? Please and spiteful language is not legally binding.
This is like asking what is it a concern of McDonald's if you open up your own restaurant call McDonald in your town and make it a dump