Why was it decided that Buffy should work at a burger bar?

Well, we gave Buffy the job at the Double Meat Palace, [and] we put her through a couple of different jobs, and we wanted to have her hit [rock] bottom.

We thought, "What's the most horrible job?" A job that's usually held by someone younger than her even - a yucky, dirty, very identifiable teenage job. Fast food just seemed like the perfect thing. It's stinky and greasy and you have to wear the stupid clothes, and it just seemed like the kind of thing that would give us maximum humiliation value.

Also, it's unnaturally creepy - a fast food place is a naturally creepy setting. There is blood built into the job, just like her other job, so it does seem like a natural for her.