While
@Ruin 's comment might be plausible, I would be also believe it's an attempt to reduce the likelihood of falling for a quick change scam -- especially for those places that hire teens that work behind the counter or cash register.
Years ago, the local ice cream place hired high school students to work part time over the summer. In response to an increase of customers paying for $1-$2 orders with $50 or $100 bills they started posting notices at the windows stating they would no longer accept anything higher than a $20 bill unless the order exceeded that amount.
A piss-off tangentially related to
@keyboredsm4shthe2nd 's post was when my neighborhood used to have annual garage sales. Fifteen minutes before the official start time, and usually before most neighbors had their stuff outside, dealers would arrive and ask to buy as much as possible for as little as possible -- usually for less than the already low prices on stuff (think 10 cents for something already marked 50). On one occasion, I recall one of my parents calling out a suspected dealer and telling them the polite version of GTFO.