Our culture is basically that we should be able to control ourselves and our eating. Being fat brings judgement but also if we tell others that we can't just "eat less / move more" and have to rely on a chemical to lose weight, it reinforces the belief non-fat people have which is that fat people have no self control, and then there will be the judgement of "well she had to rely on a drug to lose weight, couldn't do it by herself and just eat normally like the rest of us" - i.e. that it's taking the "easy option" rather than working at it, hence the cheating conclusion.
Basically, there's a lot of misunderstanding about why people become obese and also why it's so hard to lose weight once you are obese. Some (fat or formerly fat) people are confident and happily advocate and educate others when these misunderstandings come to the fore; but for a lot of us, it is an additional battle (when our self-esteem is already pretty rubbish / we already feel shame from the aforementioned cultural beliefs about fat people), that would be too much to face.
I'm keeping quiet about MJ because I don't need that judgement on top of everything else going on in my life. I know I "shouldn't" have to rely on a new and relatively untested medication to get my weight on track; but the reality is that I've not succeeded for the last 20 years in any of the other hundreds of ways I've tried, so this gives me hope. I don't want to risk anyone dissing that hope. I don't want to answer questions about how I can afford it, or field comments about me putting it all back on again afterwards. I'm doing it for me, so nobody else needs to know and I don't need to risk finding out what anyone else would think about me doing it this way either.