Its exactly this I believe. The RMAH makes it so that item-duping means someone could potentially make a shitload of money from exploiting. Also, if your account gets hacked, its your money. And we've seen through current phishing trends with banks, that even though a customer will give their details away, the banks are always to blame and the customer rarely admits to responding to phishing. This means more reputational risk and forces Blizzard to make the system as secure as they possibly can.
Its also true that they want to monitor the heck out of every game session to ensure legitimacy is upheld at all times. Having an offline mode means you would need to have a 100% offline client, which may be susceptible to hacking/modding which may lead to unfair/illegal activities giving someone an advantage. Then all they do is go online and sell all the "offline-hacked-version" gear they got.
Thats how I understand it atleast.. It probably could've been designed so that it had a smaller impact on bandwidth usage (or anything that increased latency) but hey, we're not Blizzard. Also, it seems we're not alone in this issue. There are people who get good latency that are complaining about always online and AU/NZ players are also experiencing lag to US servers.
ALL IN ALL, its still beta, and the servers are still in the US and getting rammed hard at the moment.. We could very well see "brand-new" hardware, in larger amounts, in better locations for us SA players, once its released. Till the day the game goes live in SA and someone gives it a shot, we don't know.