There's actually a really good explanation for this.

This is a mixing pump. It mixes ethanol into the fuel. Because the mixing happens before the part of the pump that measures how much is being dispensed, you need at least a few gallons to fully flush things out and get somewhere close to what you're actually buying.

Nobody is going to come arrest you if you buy 2 gallons of gas. But the gas you get me not be the mix you wanted.