Slim people don't qualify for these drugs, because they obviously don't need them, no matter who's paying for them.
And I'm sure most slim people are happy about that and not inclined to moan about purely imaginary "cheating" by morbidly obese people who clearly do need to lose weight. The effort required by reasonably slim people to stay that way is far, far less than the effort required to shed massive amounts of weight after years of chronic obesity.
The injections help these people stick to a weight loss diet - it's not some magic "quick fix", they have to do massive dieting successfully for a long time to shed all that weight.
In many cases, the drugs make this feasible when previously it seemed impossible.
Obviously, Randian-style "libertarians" will regard anyone not personally paying full price for their medicines as "cheating", as they are opposed to all publicly subsided health measures.
But I'm happy to admit in real life, and here, that I regard such people as basically evil.