I see this a little differently. Patriots won't wear that particular shirt, because it is associated with the shooter, but those who seek it and wear it are only advertising their callous disregard or feelings of solidarity with the shooter.
Let them advertise, but check to see if they got the shirt for donating to a veterans support group (I think I got one like that a few years ago, but don't recall which group sent it to me), or whether they just bought it.