The Unicode Consortium did the same to me for a totally unmonetized hobby project.
Of course sometimes mark holders will avoid going after the bigger fish where possible, just in case, unless they have a very strong case (obvious deliberate misuse of the mark).
Not legal advice. Personal opinion, in no way relates to my employment.
If someone is taking something from you, even for free or with payment in "recognition dollars", trade is happening.
And legally required or not, publicly acting to defend the mark may put off others using something similar which is presumably the desire. A bit of a bully-boy tactic perhaps, but again I'm stating the fact rather than judging it.