Catherine,
it is true that a proportion of Type 2's have the condition through no fault of their own and for whom weight loss and lifestyle changes are not going to have much effect (if any).
There are, however, a large number of Type 2's and those with pre-diabetes and insulin resistance who are overweight and obese and their weight is contributing to their condition. From a public health education perspective, it makes sense to target the blissfully ignorant majority in the knowledge that the few who are unfortunate to have heeded the health warnings but still have Type 2 Diabetes can be advised individually by their healthcare team.
While incomplete, the current message is the clearest message they can give and one that might actually get through to at least some of the population and prompt action. To complicate the message would just give people an excuse.