Actually more of the profit comes from people spending money inside the parks, not just getting them into the parks.
From Sea Worlds stand point, these stranded tourist are still going to spend money to eat and a small amount of money for entertainment during the day. If they let them in the gates for free, they'll still buy lunch and possibly a souvineer or two. This is also why all the theme parks gives their employees so many comp tickets to get friends and family in free.
That's just the business side of it all. But it's still a nice gesture, and one I'd be jumping all over if I was one of the stranded.