I hear about this often, and it happened to my sister's Macbook as well (to the point it was obviously deformed and they tried to tell her she dropped it).
Is this a case of people being more likely to complain about it because Macbooks are generally much more expensive than other laptops, or does Apple really have a battery problem with these devices?
Lithium ion batteries tend to swell when they get old. It's pretty universal. I've replaced the battery in old UPSs as well a very old 4G iPod (in which the swollen battery was keeping the clickwheel from working properly).
I'm surprised that the Apple people tried to make it seem like it was your sister's fault. In both of my cases, I went to the Apple Store and the batteries were replaced without hassle, without charge, and out of AppleCare.
Once I didn't even have an appointment; I just walked in with it and they went, "Eeeew! We'll take care of that for you. Stand right there."
In their defense, it was one manager or something, and the laptop was grossly deformed. I assume it was the battery, as I'm not sure of any other explanation. It was probably 6-7 years ago now, so the details are hazy, but I do recall her being upset and recalling how she actually got into an argument in the Apple store for a good 15 minutes before they finally accepted it. I'm not sure if they finally accepted it because they gave up, or someone looked closer and realized there were no marks on corners/sides to suggest a drop but more towards the center there definitely was deformation of the case. IIRC the tech that initially examined it was fine, but a manager came out after they took it in back.
I had a Macbook experience similar symptoms and it turned out to be pressure being exerted from the swollen battery. Twice.