You mention the media, imagine the outcry from them, and the resulting PR disaster if they didn't close?
I honestly don't think there would be any media outcry. Until I read this thread, the idea that Disney might be closing hadn't even crossed my mind so I doubt the world's press and social media would be too interested when they have much more significant stories to be focussing on.
When the 7/7 bombings happened, Alton and Thorpe didn't close. America's Disney parks did close on 9/11 but that was for security reasons and they were open the very next day. I doubt many other American parks closed at all. In these cases, there was no media backlash for lack of compassion.
Personally, I find the idea of external real world events affecting park operations to be a bit perverse. Theme parks in my mind are supposed to be an escape and as such should exist in their own little bubble. You go there to take your mind off the news and the everyday grind. Surely, if the places that people go to relax and have fun shut down, those responsible for these attacks have in some way succeeded in changing our way of life even if it's only for a few days.
I think that London after the 7/7 bombings showed extraordinary resolve. The next day, people boarded the tube as usual. Shops, restaurants and attractions were still open and after a short while, life was back to normal. It was no less respectful towards the victims, there were moments of silence held and memorial services. But the key thing was that London as a city shrugged off the attack and because of this it didn't work. Today, you just use the tube or the busses without even thinking about it, just as you could before.
Unfortunately, I don't think today's media circus allows for such a sensible reaction to pan out if an attack happens in any western city.
Can't really compare a building collapse to multiple, coordinated mass killings either.
The outcome is no less devastating to the families and friends of the victims. When it comes to disasters on this scale, surely it's the actual loss of life that is tragic, not the mechanism through which it happened. France has experienced a number of disasters over the last decade, the most obvious example being flight 447 which killed far more people. But for some reason, because this can't be attributed to a nominal bogeyman, it's somehow perceived to be not as bad.
The worst thing we can do is turn the people responsible for this into monsters. They're not, they're just idiots who have had stupid ideas put in their heads.