I believe the "docs" are subjectively set by the individual dealer. This is part of the trend where dealers make additional $ after the vehicle price is arrived at. Buyers are much more educated now on invoice prices (thx to Edmunds, etc). This leverage gives them a good idea how much the dealer is making on the deal. My experience is that dealers are now making much of their $ after arriving at the vehicle selling price (although not so much the case with cars selling >= msrp). The buyer has a tendency to "let his/her guard down" after haggling to what they think is the final price (and dealers know this). It is then that the vultures move in, pushing accessories, window vin etching, alarms, ext warrantees - all with well practiced sales pitches. The doc seems to be one of these practices.
You think you've sealed the deal, & now have your heart set on the car. Not many buyer are then going to nix the deal when then they learn of the doc and/or administrative fee, only when signing final paper work. An extra $100-$400 per car (for essentially 5 minutes of filling out paperwork) adds up quickly to a dealer.
Moral of the story: before haggling out final vehicle price, ask dealer what the admin costs, prep costs, etc. are. You want to know what your "out the door cost" is.
Two cars ago I was scammed (illegally I later found) by the finance guy. After offering a good rate of 6%, he told me he had one bank who would knock another point off- to 5% if I purchased the extended warrantee ($1000). Over life of loan, savings would be $650. Thus I was getting 100k warrantee for $350. A "no-brainer" but an illegal practice, and you know the slime got a good commission on the warrantee sale.