It probably revolves around the fact that they design for the lowest-common denominator. It's the majority of people that believe Apple when they say multi-tasking reduces battery life by 80%. They proceed to thank Apple for not allowing multi-tasking and experiencing horrible battery life.
Alternatively, there are non-idiots who will realize it is not multi-tasking that drains the battery, but rather keeping an IM client open which utilizes a continuous data connection. They understand that it would count as usage time, and fully expect the battery to be dead within 9 hours (equivalent to wifi browsing).