In fact, Blizzard makes a lot of money from this.
They hold mass bannings, ban accounts, not IP addresses, so the botters make new accounts and pay for their accounts, so Blizzard makes big bucks.
World of Warcraft does have a two (or three) week try out period.
I heard it's possible to get an account up to level 80 in that time, mining gold along the way and selling that, and finally selling the account for big bucks!
Anyhow, these are all rumors I heard, don't take 'em from me. I don't even play World of Warcraft!
I don't know about Blizzard, but Jagex (the makers of Runescape) had a whole bunch of banks gang up on them and demand that they take care of gold farming.... or they would lose the ability to use credit cards for payment of their services. Essentially the charge-backs and fraud rates (from stolen credit cards and other schemes to defraud banks) were so high that a formal investigation as to if the video game company itself was a criminal organization had started.
Such a threat forced the company to take a hard line on the issue or face a very real possibility of bankruptcy. There were several changes made to the game explicitly to get that gold farming put under control.
It turns out that the potential for money laundering and some other very real criminal organizations were involved with the gold farming too that made this more than simply fraud but something that encouraged other kinds of problems that got Scotland Yard involved with as well. There have also been a couple of players that have died over the issue (and subsequent lawsuits against the video game maker from those deaths). Basically it is a huge legal mess that their only protection was to do much more than pay lip service against gold farming.
I also know that other video game companies (MMORPG makers) have been contacted by banks about this issue and have demanded similar kinds of actions to be taken, and Blizzard is one of those who have at least made some efforts as a result of that happening too. It certainly is a blatant falsehood that the MMORPG companies involved are making a profit off of this activity and in fact it turns out in the long run to be a substantial loss of income for them instead.
Some virtual environments like Second Life and There have openly embraced "real world money", but even they have some fraud problems. If you are going to engage in trading game gold, I would strongly recommend that you stick with games that permit the activity. There certainly are enough games that permit it to remove the need for now to engage in that kind of activity for those games that discourage this trade or prohibit it entirely. It is possible to be involved with a criminal investigation if you directly engage in these kind of trades for games that prohibit trades.