BBM on the BB devices used efficient transport separate from the normal IP support of the network, which is why it was viewed as premium service by the carriers

BBM on the BB devices used efficient transport separate from the normal IP support of the network, which is why it was viewed as premium service by the carriers. iOS does something similar as a transport for APNs (and thus iMessages) with two important differences: Apple has somehow managed to convince most carriers to not bill that as premium service and can sidestep the carriers that would want to bill that as a premium service (that is the core of what the “Additional rates may apply” message in the iOS activation/onboarding process refers to, it involves few round-trips to Apple servers over SMS). BBM on Android did not do any kind of these optimizations and thus was somewhat of a battery hog (that would not fly on iPhone, so I assume it used APNS there).