It sounds like a sweet deal doesn't it. Someone pays you to visit these exotic locales, try their food and write up a book about your experiences. As much as this sounds like a dream come true, it isn't as easy as it might seem.
Many travel books, magazines and web sites are looking for people who have already been there. If they are writing a book or want a story about Disneyworld restaurants, they want someone that has been there, do some research and write something up. These places don't want to pay a contract writer to visit these places.
You can become an employee of these magazines and other places that end up writing books, but then you have to be able to leave for who knows where on a moments notice. College students on summer break might be able to get an internship or something to become a travel writer as well.
The other way a travel book gets written is that pitch is given to a publishing company and they say Ok. They provide with an advance to write the book and a deadline. The rest is up to you. Wait, you didn't actually think these people just got free trips did you?
The writer has to do all the legwork, contacting the places and most of the time has to pay for their own airfare and hotel expenses. They might get a discount and meals comped by the hotel or restaurant, but its not a free ride.