As explained
here, I guess it all boils down to one simple question we all need to ask ourselves.... "Do I really want to avail of the carrier's cellular data (internet) services with my smartphone?"
If the answer is
No, then go to the carrier and have them disable (unprovision) your account for that service.
If the answer is
Yes, then it remains the subscriber's responsibility to watch their usage, either knowingly or unknowingly.
I know the above and the following statements I will be making will get some mixed reactions, some I anticipate to be violent in nature

. Please be advised that this is just my opinion and does not in any way reflect the opinions of this site and its members/owners
Not trying to sound cynical or non-emphatizing, but we all own iPhones because of its connectivity, among others, features. As smartphone (of which the iPhone belongs to this class) users, the reason we bought them is because we wanted to stay connected no matter where we are, being able to send/receive email, surf, do yahoo messaging, update our Facebook status"es", harvest on Farmville etc.
Furthermore, the iPhone, first and foremost is and always has been marketed as an internet device, ergo, one of it's main function is to ensure that you have that (and it is doing a good job at it if you ask me), hence its heavy dependence on the cellular data network. The wi-fi feature is an "added bonus" so to speak, to serve as an alternate connectivity option.
"Always-on" connectivity is primarily being handled thru the cellular data network and that is the main design of the iPhone radio logic, vis-a-vis the behavior of it switching and connecting to the cellular data network when it goes to sleep even if you're in a wi-fi covered area.
It is unfortunate though that cellular data services of our local carriers are not as "affordable" or as flexible as compared to other countries.