Skinny bashing

This Sunday I went to the baby shower of a friend from pharmacy school.
(This baby shower, by the way, had me in frantic crochet mode. I put off starting her baby's sweater until this week, so the whole drive to Manchac and back I was speed-crocheting to get it done).

Baby showers don't generally rock my socks off, just because I find it very hard to feign interest in baby gifts. How many "awwww"s do I have to generate for decorative bottles and onesies with ponies on them? At least at bridal showers I can steal decorating ideas.
But it was good to see some pharmacy school friends I hadn't seen in awhile, especially as there was some super juicy gossip about one of our associates.

This gal...
- Got married to an ex-boyfriend and got pregnant while in pharmacy school.
- While pregnant, had an affair with her also-married boss. 
- Which caused bosses' wife to commit suicide
- Gal did not yet divorce husband, but
- Gal is now pregnant with bosses' child.

Good gosh. I'm speechless.
Anyway, back to the shower. Like many social events in Louisiana, this one had it's fair share of overweight and obese attendees. As in, most of them. Louisiana isn't doing so well as far as health and weight and diet go.
And something occurred that I've observed a lot at these events. As the cake was being cut, one guest - slender - declined cake. She was instantly showered with abuse. Actually, the verbal assaults started before she declined: When asking her if she wanted cake, the mother of the honoree called out, "Hey skinny! I know you could use some cake!"
When the girl said she didn't want any, all I could hear was sarcasm and insults.

What? Are you trying to lose weight?
Good grief, you need to eat some cake!
Oh brother, make us all look fat, go ahead!
You can eat it just this once, geeze, it won't kill you!
Yeah, you look like you never tasted cake in your life! 
Pfft, she probably just wants to lick the knife after you're done serving.

I don't know when this nonsense became acceptable, but it's downright commonplace in public. The poor girl was like, um, I just don't like white cake. Not only is it ok for her to pass up cake, it's ok to be skinny! It's not a crime! Yet it isn't just showers where I see this. Online forums claim that "All REAL women have cellulite" and praise companies who use overweight models, saying they are REAL women. When celebrities lose weight they're criticized, even if they are still a healthy weight. Choosing healthy options over sugary snacks is praised if it's done by someone trying to lose weight, but if you're already thin and skip the cake you get snide remarks.
When did we start condoning skinny bashing? Imagine if the situation at the shower was reversed. I can just see the reaction if an overweight guest accepted cake and everyone condemned her for it. If people told her, "You're already fat, have an apple for once!" or muttered, "Watch, she probably wants the piece with the most icing" - well, we'd be appalled. But we can do the exact same thing to our thin friends.
What gives?
Have you seen this? Have you responded?