Why are some people sooo stupid?

Last night, I stopped to get some Caribbean food at one of my favourite Caribbean restaurants, on my way the C&C music factory for an evening of Amazing Race and chit chat. The line-up at this place is often out the door, and every transaction seems to take forever. So when you walk in and you've only got 5 minutes to spare and there are only 3 people in front of you in line, in most fast-food type restaurants, you'd imagine that would be enough time - not at this place. Now, normally the people in front of you in line are pretty familliar with the Caribbean cuisine, so you don't have to worry about them holding the line up too long with too many questions, they know what they want and they ask for it in the shortest possible amount of time. But, the kiss of death, the worst possible thing is to be stuck in line behind some mangia-cake who sometimes goes to this restaurant and his friends who've never eaten anywhere but McDonalds. That's what happened last night.
Dude who's been there before orders a really big but relatively standard meal, chicken, beans and rice, gravy, a dumpling, a patty, oh and 3 plantains. His two friends agree to order the same thing as him. And then the girl pipes up, "I don't want any plankton on mine though" (she meant plantain).
Everyone stares at her and her friend and the lady behind the counter correct her, and then 30 seconds later she says, "Oh, but don't forget to put the plankton on the other one." Her friend attempts to correct her again and she replies, "What? I just don't want any plankton!"

I suppose it's possible that she's hard of hearing, and that's why (a) she couldn't figure out what was going on and (b) why she was so loud, but I couldn't help but notice that she had a perfect manicure and one of those TNA jackets on - I think they're going to be the new Invicta backpack.