You may not be understanding the actual issue. It isn't whether or not he was invited to be there. If after being invited, he'd gone and spent time with the family at the cemetery, we wouldn't even be talking about it.
The issue is that he and his campaign staff went to the cemetery, took photos and video, and then used them for his campaign, which is expressly prohibited by federal law. Not only that, but his campaign staff shoved a cemetery worker. Plus, everyone was told beforehand about how using Arlington National Cemetery for political campaigns is illegal, yet they plowed ahead and did it anyway.
It's sad that the impression I've gotten from Trump supporters is along the lines of "so what" or "who cares", which brings me back to the question posed in this thread. Unfortunately, it looks like the answer for some of you is "nothing".