They probably assume that people who buy ready meals are, like you, people who want to save time - and that people who want to save time are also, logically, people with time-saving devices such as microwave ovens.
Almost every kitchen has a microwave oven these days, so it doesn't seem unreasonable to me that food manufacturers should assume that their customers will posses one. (I haven't got a fridge but I don't complain when I read 'store in a fridge' on product packaging; I simply accept that I'm in a very small minority).
Microwave ovens are great for heating ready meals and, for me, especially for defrosting stuff from the freezer quickly. (Yes, I do have a small freezer, even though I've not got a fridge). They're also the very best way of cooking some foods, such as firm-textured fish (e.g. salmon steaks or small trout).