This.
Also, Blizzard is a business. The optional services are making them money, at least some of which goes towards maintaining the game and paying their employees. Sure, some of that money goes into the pockets of their rich higher-up boss guys, but that's capitalism for ya. They're allowed to do that and will probably continue to do so as long as we players pay for the services.
Also, free transfers would destroy any remaining traces of community that the servers have. That cool guild you joined last week? They all transferred and left you behind. That blood elf pally that tanked your 5-mans? He's a dwarf now. The guys who buy all your overpriced stuff in the AH? Transferred.