Hollywood Reporter states that the item is sold in Target, but that after approaching Mad Engine DC Comics received no response for almost three weeks "in an effort to allow the Infringing T-Shirt to remain available for sale through Father's Day". DC Comics states that their "copyrighted Shield Design consists of a bordered five-sided shield in red and yellow, with the text inside the shield sized and positioned according to the proportions and shape of the shield. The shield design on Defendant's Infringing T-Shirt incorporates each of these elements."
And being a central Marvel licensee probably doesn't help. Also, this shield design have been used in a variety of forms by a variety of folk. Could this be a shot across the bows?