I want to start by saying that the answer you seek is not in the passage you quote. The sentence of interest here,
Markets are nurtured by global companies in the tourism system.
is delivered as a bald assertion of fact that is in no way explicated or explained. It is delivered, and then taken as a premise for all that follows. What follows is, at most, indirectly informative.
Here's the guesses I would make about what's being said, as a native speaker. I can't tell you they are right, because the information is not available to me to know whether or not they are right.
I would assume that the sentence should not be taken strictly literally, in a vacuum: if we take it a face value it's a statement about "markets", which seems clearly to be wrong. This is not a passage of economics theory, discussing theoretical "markets". It's saying something about tourism. I know there's theories about markets that predicate everything on labor, and others that predicate everything on currencies, and others that predicate everything on manufacturing, but I have yet to hear an economic theory that explains modern society in terms of tourism. So that's probably not what's happening.
I'm guessing the missing context here is that it's talking about markets for tourism. Which if we substitute gives us:
Markets for tourism are nurtured by global companies in the tourism system.
That actually now says something interesting and informative: it's saying that markets for tourism aren't just natural phenomena. They're deliberately cultivated by businesses that stand to profit by doing so. Which, yes, makes sense: companies which sell tourism (travel, accommodations, tours, etc.) have to promote, by means of marketing and physical development, tourism markets, meaning regions or (in tourism industry jargon) "destinations".
For instance, if an airline wanted to increase the profitability of their flights to a new location, they might:
• Advertise the location ("Now with three flights daily to Northern Erewhon!")
• Offer price incentives ("Sale! $99 round trip to Northern Erewhon!")
• Make deals with hotels and restaurants in the area, to offer package deals ("$1000 three night stay in Northern Erewhon!")
• Encourage the local government to improve the roads, the transportation between the airport and the sites of interest, the sites themselves.
• Fund travel writers to write newspaper, magazine, and blog articles about how wonderful it is to visit the location.
What "nurture" here means is something closer to "cultivate": take active steps to encourage the growth of.
So I'm thinking that the author's point here is that green tourism will not happen without these companies, which have so much to do with whether a tourism market thrives or not.
In a sense, what the author is saying (I surmise) is, "Look, green tourism is an important idea. But people don't just go to Borneo because they up and decided one day to go to Borneo. They go to Borneo because some big international tourism industry company whipped out a checkbook at some point and paid for an ad for flights to Borneo; and some company put down the order for a "VACATION IN BORNEO" website to tell everyone in the world how much fun Borneo is; and some company sunk real money into building hotels in Borneo so tourists would have a place to stay and other companies sunk real money into building the infrastructure for tours in Borneo so tourists would have things to do in Borneo; and all the rest that turned Borneo into a desirable tourist destination. If all that hadn't happened, nobody would want to vacation in Borneo; most Westerners wouldn't even have heard of it. So appreciate that what looks like a wild jungle of a tourism market in Borneo is nothing of the sort: it's a carefully cultivated garden, and the companies that brought it to fruition feel mighty invested in it. If you want to promote green tourism, you will find a way to work with them, or they will find a way to work against you."