Sorry but IMO that completely misses the point between official royals - Queen Elizabeth II was the UK's Head of State visiting her people at what was effectively a national disaster. The reason Heads of State or royals (official working royals) visit is to represent the rest of the country and because the rest of the country can not. They visited a sports centre being used by the emergency services, families and some of those who escaped. They stayed for a relatively short amount of time and focussed on meeting those emergency services who had been working around the clock. The Prime Minister at the time wouldn't visit due to security concerns (enough said about that) and was getting stick for not visiting, the Queen went because she was Head of State (and as BP used to define it - "Head of Nation") so she went to show those who had worked and risked their lives how grateful the nation was, as well as to show the survivors and their families how shocked and saddened the nation was- she went on behalf of everyone in the UKWhen the Grenfell Tower disaster happened, multiple Royals made visits include QEII and Prince William. Would you call those visits "Disaster tourism", especially since those Royals arrived empty handed but with camera in tow?
Harry and Meghan are not - in this context - royals. They may have royal titles and Harry may be the son of a King but they are not royals who represent anyone but themselves. They are not there on behalf of any nation, people or crown.
Again, personally I have no issue with them helping out- it is a noble thing to do. I also have no issue with them being captured doing so on camera - everywhere is surrounded by cameras at the moment it seems. But I have issue with a mayor taking time out to show them around and with them being given access to areas others don't. That might be fine if they were on a visit as official royals but they aren't, there is no need to give them special access and tours by officials who should have much better things to do.
They also visited two days after the fire - not while it was still burning but after the worst was out and when visiting meant not taking up time of people still fighting an active fire.
I would also add, if you want to look at a personal level, taking out royalty - the Queen and William travelled 18 minutes, just over 4 miles from Buckingham Palace to Grenfell - Harry and Meghan travelled 1hr 30+ and 95+ miles to visit Pasadena. Thus one is inextricably more local than the other making those who visited more connected and more local than the other.