They want to cover their interests. Different people can react to a VLCD + medication differently and there is NO way to predict this. Some people are fine, but - with medication you should be careful.
My OH's mum, for example, is bi-polar and is sensitive to fluctuations of her medication's concentration within her body. She would not do this diet for this reason.
The increased fluid intake on this diet may dilute your dosage and bring unwanted side-effects.
I used to be on anti-psychotic medication when I was 19 and when I came off it - the withdrawal effects were rather spectacular for a few months. Imagine your body not getting what it's supposed to because it's all getting wee'd out! Not nice.
So S&S are looking out for you as well as looking after their business!