This is the OPM mentality at it. I rent way, way too many cars in a year (in fact, if I take out our cross-country drives for our Ohio move, I've put more miles in rentals than my own cars this year!), and I only do a cursory check of the reciept. A $2 swing in price wouldn't set off any alarms with me (as I'm used to varying local and state taxes), so the firm would get the $2 per car without many even noticing. And most people, if they did notice, would say "Screw it" and just pay it.
For most of my rentals, that's an effective 1% bump pre-tax.
Sigh.
Cheers,
-Andrew