So you are telling me that you've never had someone sit down at a table while the others are getting the food? Saving a table is not like cutting in line.
It's exactly equal to cutting in line. The restaurant provides tables so its diners can have a safe, convenient, comfortable place to partake of the food they've purchased at that location. They got to the table first. First come, first serve.
That belies the entire purpose of the restaurant, as well as the definition of the word "served" in this exact context. The person who rushes to a table to hold it twenty minutes before their food arrives is NOT being served. That's basic common sense.
It's actually basic "me first, the heck with the rest of the world" nonsense 
Frankly, even if it was just that one lone person sitting at the table with no intention of eating but just to take a break in the air conditioning, that's their right. Is it somewhat inconsiderate to take up a 4-top table when it's just you? Yes, but again, they got their first and they have every right to sit down and rest for awhile.
While I disagree any person has the 'right' to walk into a restaurant just to sit in air conditioning, the restaurant patron's right to sit at that table and eat the food they purchased at that restaurant is equal or greater. It's annoying, but if you sat down next to me while I was waiting for my family to get the food, I'd be freaking ticked, and I may just jostle you off the seat.
Respectfully, since you would not yet be a customer of that restaurant but merely a potential customer, while the poster you would be jostling IS a customer - having purchased food - in the case of an unprovoked altercation, likely you would be at fault and she would be the victim. Remember, it's not your table OR her table. It belongs to Walt Disney World - and possession ISN'T 9/10 of the law. However, if I'm sitting there by myself taking a break in the air conditioning, I would have no problem with someone asking if they could sit there. Sure, you can have the other 3 seats that are left at the table.
Mighty generous of you.