Would it not prevent others from making money off their dead son's name?
No. They trademarked it as "I AM TRAYVON" and "JUSTICE FOR TRAYVON" for use on "Digital materials, namely, CDs and DVDs featuring TRAYVON MARTIN; Digital media, namely, pre-recorded DVDs, downloadable audio and video recordings, and CDs featuring and promoting TRAYVON MARTIN; Digital media, namely, pre-recorded video cassettes, digital video discs, digital versatile discs, downloadable audio and video recordings, DVDs, and high definition digital discs featuring TRAYVON MARTIN; Digital media, namely, CDs and DVDs featuring TRAYVON MARTIN; DVD cases; DVD sleeves; DVDs featuring TRAYVON MARTIN"
Others have TMd various combinations of his name for hoodies, a charitable organization, and Obama's quote "If I had a son, he'd look like Trayvon Martin" as a slogan.
BTW, the "Justice For Trayvon Martin Foundation" was handled by the same attorney as the attorney on record as the one his family used to TM the name.