Theme parks are expensive just to visit and spend a day at. Add to that the costs of traveling. When theme park attendance is up it means people aren't too worried about their finances. They have extra money so they treat themselves to a visit to the theme parks.
Blackkid's comment in No, yes sometimes people do spend huge amounts of money unwisely and out of fear of loss. However, people are more likely to respond to being deep in debt or to having little money by not doing things like going to theme parks, so an increase in numbers is a good economic signal. Theme parks require planning, and are usually undertaken with the whole family or with friends (so the "Are you sure this is a good idea?" discussion would be more likely to come up and stop the trip if there isn't enough money). When people feel down and out and spend unwisely it's usually with trips to the local bars, not far away theme parks.
Increases in theme park attendance is a sign that visitors have more money to spend on entertainment, and that is a good signal to economists of a stronger marketplace. If the surge in attendance is from guests on vacation or outside of the area of the park, tourism and related businesses will reap benefits. If the surge includes local residents, it is a sign that the local economy is stronger, and that more disposable income is available for the locals to spend in their communities.
Yes, economists should consider a surge in theme park attendance a good economic signal, because theme parks are not someplace that people go when they do not have a lot of extra money. Theme park spending is a sign that people have money to spend on fun things again. It's a sign that things are improving.
Theme parks are not cheap, so do not fall into the category of "little luxuries" that people treat themselves with in otherwise poor circumstances. When theme park attendance is up, people have disposable income to spend on entertainment and are feeling optimistic. It is, therefore, a great sign that people are feeling better off and able to spend money on more than just the bare necessities.
I think economists are trying to look for good news in a society that barely functions due to the fact that many people are still working minimum wage jobs as a career. The economy will not fix itself. The people holding the money are going to have to let it go or this will never correct itself.
The first interval in which a person spends a lot of money is prosperity however the second interval is stress and poverty. The threat of loss can and does drive people to spend money inappropriately so a surge in luxuries of any sort needs greater attention than simply the assumption that everything is dandy.